Add specs for ClasspathEntityScanner and ReflectionUtils

[borinquenkid]

Summary

• Add ReflectionUtilsSpec covering isMethodOverriddenFromParent: a genuinely overridden public method, a method unique to the subclass, a method whose declaring class has no superclass (e.g. Object), and a package-private method that only shadows a same-named parent method.
• Expand ClasspathEntityScannerSpec from a single happy-path test to cover jakarta.persistence.Entity detection, non-entity classes being excluded, empty package input, de-duplication across a repeated package, combining results across multiple packages, and the previously-untested "ignored package" branch (com, org, etc. are skipped rather than scanned).
• Minor cleanup in ReflectionUtils.isMethodOverriddenFromParent: drop a dead null check — Class#getMethod never returns null, it throws NoSuchMethodException instead.

Test plan

• ./gradlew :grails-datamapping-core:test --tests "org.grails.datastore.gorm.utils.ClasspathEntityScannerSpec" --tests "org.grails.datastore.gorm.utils.ReflectionUtilsSpec"
• ./gradlew :grails-datamapping-core:test :grails-datamapping-core:codenarcMain :grails-datamapping-core:codenarcTest

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.4311%. Comparing base (75cd618) to head (c87627e).

Additional details and impacted files

@@                Coverage Diff                 @@
##                8.1.x     #16188        +/-   ##
==================================================
+ Coverage     53.4149%   53.4311%   +0.0162%     
- Complexity      19459      19466         +7     
==================================================
  Files            2081       2081                
  Lines           98993      98991         -2     
  Branches        17361      17360         -1     
==================================================
+ Hits            52877      52892        +15     
+ Misses          38566      38552        -14     
+ Partials         7550       7547         -3     

Files with missing lines	Coverage Δ
...g/grails/datastore/gorm/utils/ReflectionUtils.java	87.5000% <100.0000%> (+87.5000%)	⬆️

... and 5 files with indirect coverage changes

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[testlens-app]

✅ All tests passed ✅

🏷️ Commit: c87627e
▶️ Tests: 63321 executed
⚪️ Checks: 77/77 completed

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