test: stress bounded native pattern cache

[jbachorik]

Summary

Adds deterministic high-cardinality stress coverage for the bounded native pattern compiler cache.

• 128 unique sequential native admissions and 400 concurrent admissions remain bounded at configured capacity.
• Eviction is proven by recompiling the oldest key with a new identity.
• Legacy static compiler caches remain unchanged; evicted-but-held patterns still match guarded input without materialization.

Validation

• ./gradlew :reggie-runtime:test --tests com.datadoghq.reggie.runtime.ReggieCompiledPatternCacheCardinalityTest
• ./gradlew :reggie-runtime:test
• ./gradlew spotlessApply
• final implementation-plan validation: clean
• final independent code review: clean

[Copilot]

Pull request overview

Adds deterministic stress tests to validate that the instance-owned, bounded native pattern compiler cache remains capped at its configured capacity under high-cardinality sequential and concurrent admissions, while ensuring legacy RuntimeCompiler static caches are not mutated.

Changes:

• Introduces a sequential high-cardinality test proving cache plateauing and eviction via identity change on recompilation of the oldest key.
• Adds a concurrent admission stress test ensuring the cache remains bounded and legacy caches remain unchanged.
• Verifies evicted-but-still-referenced compiled patterns can still match without materializing the input (guarded CharSequence).

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.5%. Comparing base (c3fcc3e) to head (61206a8).

Additional details and impacted files

@@ Coverage Diff @@## agent/logs-reggie-l1j #120 +/- ##
=======================================================
Coverage 84.5% 84.5% Complexity 1 1 =======================================================
Files 156 156 Lines 43277 43277 Branches 5910 5910 =======================================================
Hits 36571 36571 Misses 4913 4913 Partials 1793 1793 

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