test(processing): avoid JDK crash in schemaless segment test

[FrankChen021]

Summary

• Migrate SchemalessTestSimpleTest from class-level JUnit 4 parameterization to method-level JUnit 5 @ParameterizedTest.
• Create fresh incremental and merged fixtures for each method-source invocation.
• Use try-with-resources in each test method so every test invocation owns and closes its Segment.
• Close intermediate mapped indexes while retaining the returned merged index for the owning segment.

JDK crash context

This change addresses the intermittent direct-buffer JVM crash observed in the crash reproduction PR #20028, which affected SchemalessTestSimpleTest on JDK 25.

Validation

• Processing test-compile passed.
• Focused test passed after the lifecycle change: 36 active executions; the existing disabled TopN test remains skipped.
• The equivalent lifecycle workaround was also exercised in 100 separate Maven invocations on Temurin JDK 25 with Surefire retries disabled, with no JDK crash.
• Checkstyle, SpotBugs, and git diff --check passed.

This PR is intended to validate whether explicit fixture ownership and cleanup prevent the intermittent JDK direct-buffer crash.

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