Fix VirtualThread support on multiple unmount/remount cycles

[PerfectSlayer]

What Does This Do

This PR supersedes #10887 to fix context tracking on multiple virtual thread unmount / remount cycles.

Motivation

Previously, only one mount was supported, leading to context break.

Additional Notes

This PR is a re-implementation and adds the following changes compared to the original PR report:

• Updates the Javadoc to clarify the expected lifecycle and context related operations
• Distinguishes afterTerminate / afterDone hooks
• Uses typed ContextScope to reduce unnecessary type casts
• Avoid unnecessary null checks for null-safe API calls
• Does not test remount cycle from the tests dedicated to VirtualThread and its builder API but creates a dedicated test suite
• Does not limit to test the missing behavior but tests other cases too like:
  • multiple unmount/remount cycles
  • implicit (tracing) context restored
  • multiple virtual threads unmount/remount cycles
  • sanity check without context
• Tries to trigger unmount more reliably - no dedicated API for so we can expect the test to flake
• Removes the logging correlation concern from the test cases to focus only on context tracking
• Adds thread join timeout

Contributor Checklist

• Format the title according to the contribution guidelines
• Assign the type: and (comp: or inst:) labels in addition to any other useful labels
• Avoid using close, fix, or any linking keywords when referencing an issue
  Use solves instead, and assign the PR milestone to the issue
• Update the CODEOWNERS file on source file addition, migration, or deletion
• Update public documentation with any new configuration flags or behaviors

Jira ticket: [PROJ-IDENT]

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