[FLINK-40561][changelog] Migrate flink-statebackend-changelog tests to JUnit 5 and AssertJ

[Alwaysgaurav1]

What is the purpose of the change

Under parent umbrella FLINK-25325 and issue FLINK-40561, this pull request modernizes all remaining JUnit 4 and Hamcrest test classes in the flink-statebackend-changelog module to JUnit 5 (Jupiter) and AssertJ.

Brief change log

• Migrated test classes and test methods to JUnit 5 annotations (@Test, @BeforeEach, @AfterEach, @TempDir) and package-private visibility.
• Converted parameterized test suites (ChangelogDelegateStateTest, ChangelogKeyedStateBackendTest) to Flink's @ExtendWith(ParameterizedTestExtension.class) and @TestTemplate.
• Migrated legacy @Rule TemporaryFolder to Jupiter's @TempDir Path.
• Replaced @Test(expected = ...) with AssertJ's assertThatThrownBy(...).
• Modernized all assertion statements from org.junit.Assert.* and Hamcrest matchers (containsInAnyOrder, etc.) to fluent AssertJ assertThat(...) assertions.
• Test suites covered:
  • ChangelogMetricGroupTest
  • ChangelogStateBackendLoadingTest
  • ChangelogStateDiscardTest
  • ChangelogListStateTest
  • ChangelogMapStateTest
  • ChangelogPqStateTest
  • StateChangeLoggerTestBase
  • KvStateChangeLoggerImplTest
  • PriorityQueueStateChangeLoggerImplTest
  • ChangelogDelegateStateTest
  • ChangelogKeyedStateBackendTest
  • ChangelogStateBackendTestUtils

Verifying this change

This change is a test cleanup and refactoring with zero production code changes.

• Verified all module unit tests pass (mvn test -pl flink-state-backends/flink-statebackend-changelog: 668 tests executed, 0 failures, 0 errors, 31 skipped).
• Verified Checkstyle rules pass with 0 violations (mvn checkstyle:check -pl flink-state-backends/flink-statebackend-changelog).

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• f02648b Azure: FAILURE

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