Add spec coverage for AggregateTimestampProvider

[borinquenkid]

Summary

• AggregateTimestampProvider was the only class in org.grails.datastore.gorm.timestamp without test coverage; added AggregateTimestampProviderSpec covering supportsCreating/createTimestamp delegation (single provider, first-match among multiple, none-support, no-providers-registered) and the timestampProviders getter/setter.
• Replaced timestampProviders.iterator().next() with List.getFirst() (JDK 21 SequencedCollection) in AggregateTimestampProvider.
• Minor javadoc whitespace cleanup in DefaultTimestampProvider.

Test plan

• ./gradlew :grails-datamapping-core:test --tests "org.grails.datastore.gorm.timestamp.*" — all pass
• ./gradlew :grails-datamapping-core:codeStyle — clean

🤖 Generated with Claude Code

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.4381%. Comparing base (75cd618) to head (a4218ff).

Additional details and impacted files

@@                Coverage Diff                 @@
##                8.1.x     #16164        +/-   ##
==================================================
+ Coverage     53.4149%   53.4381%   +0.0232%     
- Complexity      19459      19471        +12     
==================================================
  Files            2081       2081                
  Lines           98993      98993                
  Branches        17361      17361                
==================================================
+ Hits            52877      52900        +23     
+ Misses          38566      38544        -22     
+ Partials         7550       7549         -1     

Files with missing lines	Coverage Δ
...ore/gorm/timestamp/AggregateTimestampProvider.java	100.0000% <100.0000%> (+100.0000%)	⬆️
...store/gorm/timestamp/DefaultTimestampProvider.java	100.0000% <ø> (ø)

... and 2 files with indirect coverage changes

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[testlens-app]

✅ All tests passed ✅

🏷️ Commit: a4218ff
▶️ Tests: 62792 executed
⚪️ Checks: 77/77 completed

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