fix(neo4j): assert background thread completion in OptimisticLockingSpec

[borinquenkid]

Summary

• Follow-up to test(neo4j): give OptimisticLockingSpec's cross-thread heisenbug more headroom #16070 (merged). Copilot's review flagged that the second test's Thread.start { ... }.join(2000) can return on timeout without the background thread having actually finished, so the "same headroom rationale" comment added in test(neo4j): give OptimisticLockingSpec's cross-thread heisenbug more headroom #16070 was inaccurate there.
• Captures the thread and asserts !isAlive() after the bounded join(5000) so a slow runner fails loudly instead of silently racing the assertions that follow, instead of swallowing a potential InterruptedException and hoping for the best.

Test plan

• CI run of grails.gorm.tests.OptimisticLockingSpec in grails-data-neo4j (could not execute this module's tests locally — it's pinned to grails-gradle-plugins:6.1.2, which 404s from repo.grails.org and isn't in local caches, a pre-existing environment gap unrelated to this change)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR tightens the OptimisticLockingSpec Neo4j test to avoid silently continuing after a timed Thread.join(timeout) that may return before the background write completes, making slow CI runners fail deterministically instead of racing subsequent assertions.

Changes:

• Capture the background update thread in Test optimistic locking disabled with 'version false'.
• Replace Thread.start { ... }.join(2000) with a stored thread, join(5000), and an explicit !isAlive() completion assertion.
• Stop swallowing potential InterruptedException from the join path (now an interrupt will fail the test rather than masking it).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.1662%. Comparing base (184020c) to head (cd0502f).

Additional details and impacted files

@@                          Coverage Diff                           @@
##             feat/gorm-registry-core-impl     #16072        +/-   ##
======================================================================
+ Coverage                         52.1558%   52.1662%   +0.0104%     
- Complexity                          18408      18412         +4     
======================================================================
  Files                                2054       2054                
  Lines                               96622      96622                
  Branches                            16846      16846                
======================================================================
+ Hits                                50394      50404        +10     
+ Misses                              38835      38825        -10     
  Partials                             7393       7393                

see 3 files with indirect coverage changes

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[testlens-app]

✅ All tests passed ✅

🏷️ Commit: cd0502f
▶️ Tests: 40637 executed
⚪️ Checks: 55/55 completed

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