Fixes oppia#4100 : Added tests for PolynomialSubject (oppia#5667)

Fix oppia#4100
### New Tests for Polynomial Functionality:

* **Build Configuration Update:**
* Added a new test target `PolynomialSubjectTest` to the `BUILD.bazel`
file. This includes necessary dependencies and configurations for the
new test.

* **New Test File:**
* Created `PolynomialSubjectTest.kt` which contains various test cases
to validate polynomial operations. This includes tests for checking
invalid polynomials, constant polynomials, term counts, and polynomial
evaluations.

#### Screenshot of the passing tests.
![Screenshot from 2025-01-26
16-03-01](https://github.com/user-attachments/assets/e194d152-e4fc-4840-a30d-fa60f739ff81)



## Essential Checklist
<!-- Please tick the relevant boxes by putting an "x" in them. -->
- [x] The PR title and explanation each start with "Fix #bugnum: " (If
this PR fixes part of an issue, prefix the title with "Fix part of
#bugnum: ...".)
- [x] Any changes to
[scripts/assets](https://github.com/oppia/oppia-android/tree/develop/scripts/assets)
files have their rationale included in the PR explanation.
- [x] The PR follows the [style
guide](https://github.com/oppia/oppia-android/wiki/Coding-style-guide).
- [x] The PR does not contain any unnecessary code changes from Android
Studio
([reference](https://github.com/oppia/oppia-android/wiki/Guidance-on-submitting-a-PR#undo-unnecessary-changes)).
- [x] The PR is made from a branch that's **not** called "develop" and
is up-to-date with "develop".
- [x] The PR is **assigned** to the appropriate reviewers
([reference](https://github.com/oppia/oppia-android/wiki/Guidance-on-submitting-a-PR#clarification-regarding-assignees-and-reviewers-section)).
This pull request introduces a new set of tests for polynomial-related
functionality in the `oppia_android_test` project. The primary changes
include the addition of a new test file and corresponding updates to the
build configuration.

---------

Co-authored-by: theayushyadav11 <your.email@example.com>

scripts/assets/test_file_exemptions.textproto

@@ -4106,10 +4106,6 @@ test_file_exemption {
   exempted_file_path: "testing/src/main/java/org/oppia/android/testing/math/MathParsingErrorSubject.kt"
   test_file_not_required: true
 }
-test_file_exemption {
-  exempted_file_path: "testing/src/main/java/org/oppia/android/testing/math/PolynomialSubject.kt"
-  test_file_not_required: true
-}
 test_file_exemption {
   exempted_file_path: "testing/src/main/java/org/oppia/android/testing/math/TokenSubject.kt"
   test_file_not_required: true

testing/src/main/java/org/oppia/android/testing/math/PolynomialSubject.kt

@@ -14,8 +14,6 @@ import org.oppia.android.util.math.getConstant
 import org.oppia.android.util.math.isConstant
 import org.oppia.android.util.math.toPlainText
 
-// TODO(#4100): Add tests for this class.
-
 /**
  * Truth subject for verifying properties of [Polynomial]s.
  *

testing/src/test/java/org/oppia/android/testing/math/BUILD.bazel

@@ -67,3 +67,18 @@ oppia_android_test(
         "//third_party:robolectric_android-all",
     ],
 )
+
+oppia_android_test(
+    name = "PolynomialSubjectTest",
+    srcs = ["PolynomialSubjectTest.kt"],
+    custom_package = "org.oppia.android.testing.math",
+    test_class = "org.oppia.android.testing.math.PolynomialSubjectTest",
+    test_manifest = "//testing:test_manifest",
+    deps = [
+        "//model/src/main/proto:math_java_proto_lite",
+        "//testing/src/main/java/org/oppia/android/testing/math:polynomial_subject",
+        "//third_party:com_google_truth_truth",
+        "//third_party:junit_junit",
+        "//third_party:robolectric_android-all",
+    ],
+)

testing/src/test/java/org/oppia/android/testing/math/PolynomialSubjectTest.kt

@@ -0,0 +1,173 @@
+package org.oppia.android.testing.math
+
+import org.junit.Assert.assertThrows
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+import org.oppia.android.app.model.Polynomial
+import org.oppia.android.app.model.Real
+
+/** Tests for [PolynomialSubject]. */
+@RunWith(JUnit4::class)
+class PolynomialSubjectTest {
+
+  /** Helper function to create a polynomial term. */
+  fun createTerm(coefficient: Int, vararg variables: Pair<String, Int>): Polynomial.Term {
+    return Polynomial.Term.newBuilder()
+      .setCoefficient(Real.newBuilder().setInteger(coefficient))
+      .apply {
+        variables.forEach { (name, power) ->
+          addVariable(Polynomial.Term.Variable.newBuilder().setName(name).setPower(power))
+        }
+      }
+      .build()
+  }
+
+  /** Helper function to create a polynomial from multiple terms. */
+  fun createPolynomial(vararg terms: Polynomial.Term): Polynomial {
+    return Polynomial.newBuilder().apply {
+      terms.forEach { addTerm(it) }
+    }.build()
+  }
+
+  @Test
+  fun testPolynomialSubject_withNullPolynomial_isNotValidPolynomial() {
+    PolynomialSubject.assertThat(null).isNotValidPolynomial()
+  }
+
+  @Test
+  fun testPolynomialSubject_withNonNullPolynomial_isNotValidPolynomial_fails() {
+    val polynomial = createPolynomial(
+      createTerm(5, "x" to 1)
+    )
+    assertThrows(AssertionError::class.java) {
+      PolynomialSubject.assertThat(polynomial).isNotValidPolynomial()
+    }
+  }
+
+  @Test
+  fun testPolynomialSubject_withConstantPolynomial_isConstantThat() {
+    val constantPolynomial = createPolynomial(
+      createTerm(5)
+    )
+    PolynomialSubject.assertThat(constantPolynomial)
+      .isConstantThat()
+      .isIntegerThat()
+      .isEqualTo(5)
+  }
+
+  @Test
+  fun testPolynomialSubject_testIsConstantThat_withNonConstantPolynomial_fails() {
+    val nonConstantPolynomial = createPolynomial(
+      createTerm(5, "x" to 1)
+    )
+
+    assertThrows(AssertionError::class.java) {
+      PolynomialSubject.assertThat(nonConstantPolynomial).isConstantThat()
+    }
+  }
+
+  @Test
+  fun testPolynomialSubject_withZeroTerms_hasTermCountThatIsEqualToZero() {
+    val emptyPolynomial = Polynomial.newBuilder().build()
+    PolynomialSubject.assertThat(emptyPolynomial)
+      .hasTermCountThat()
+      .isEqualTo(0)
+  }
+
+  @Test
+  fun testPolynomialSubject_withTwoTerms_hasTermCountThatIsEqualToTwo() {
+    val multiTermPolynomial = createPolynomial(
+      createTerm(1),
+      createTerm(2)
+    )
+
+    PolynomialSubject.assertThat(multiTermPolynomial)
+      .hasTermCountThat()
+      .isEqualTo(2)
+  }
+
+  @Test
+  fun testPolynomialSubject_withValidIndex_termHasCoefficientAndvariable() {
+    val polynomial = createPolynomial(
+      createTerm(5, "x" to 2),
+      createTerm(3, "y" to 1)
+    )
+
+    PolynomialSubject.assertThat(polynomial)
+      .term(0)
+      .hasCoefficientThat()
+      .isIntegerThat()
+      .isEqualTo(5)
+
+    PolynomialSubject.assertThat(polynomial)
+      .term(0)
+      .variable(0)
+      .hasNameThat()
+      .isEqualTo("x")
+
+    PolynomialSubject.assertThat(polynomial)
+      .term(1)
+      .hasCoefficientThat()
+      .isIntegerThat()
+      .isEqualTo(3)
+  }
+
+  @Test
+  fun testPolynomialSubject_failsWithInvalidIndex() {
+    val polynomial = Polynomial.newBuilder().build()
+    assertThrows(IndexOutOfBoundsException::class.java) {
+      PolynomialSubject.assertThat(polynomial).term(0)
+    }
+  }
+
+  @Test
+  fun testPolynomialSubject_withConstantPolynomial_evaluatesToPlainText() {
+    val constantPolynomial = createPolynomial(
+      createTerm(5)
+    )
+    PolynomialSubject.assertThat(constantPolynomial)
+      .evaluatesToPlainTextThat()
+      .isEqualTo("5")
+  }
+
+  @Test
+  fun testPolynomialSubject_withComplexPolynomial_evaluatesToPlainText() {
+    val polynomial = createPolynomial(
+      createTerm(2, "x" to 2),
+      createTerm(3, "x" to 1),
+      createTerm(1)
+    )
+
+    PolynomialSubject.assertThat(polynomial)
+      .evaluatesToPlainTextThat()
+      .isEqualTo("2x^2 + 3x + 1")
+  }
+
+  @Test
+  fun testPolynomialSubject_withTwoTerms_hasVariableCountThatIsEqualToTwo() {
+    val polynomial = createPolynomial(
+      createTerm(5, "x" to 2, "y" to 1)
+    )
+
+    PolynomialSubject.assertThat(polynomial)
+      .term(0)
+      .hasVariableCountThat()
+      .isEqualTo(2)
+  }
+
+  @Test
+  fun testPolynomialSubject_polynomialTermVariable_hasExpectedDetails() {
+    val polynomial = createPolynomial(
+      createTerm(1, "x" to 3)
+    )
+
+    PolynomialSubject.assertThat(polynomial)
+      .term(0)
+      .variable(0)
+      .apply {
+        hasNameThat().isEqualTo("x")
+        hasPowerThat().isEqualTo(3)
+      }
+  }
+}