fix(search): invalidate runtime caches when config changes

[SaitejaKommi]

Summary

This PR adds automatic invalidation for in-process search caches when the global configuration object changes.

It prevents stale query builders and result processors from being reused after configuration reloads, ensuring runtime behavior always matches the active configuration.

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Related Issue

Fixes #386

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What Changed

• Added automatic cache invalidation when global config changes
• Preserved cache reuse when configuration remains unchanged
• Prevented stale query builders and result processors after reload
• Added unit tests for cache reuse and invalidation behavior

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Affected Files

• search.py
• test_search_runtime_cache.py

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Testing

Full local gate executed successfully:

• Backend unit tests ✅
• Backend integration tests ✅
• Frontend build and tests (dev) ✅
• Frontend build and tests (prod) ✅

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Notes

This is a runtime consistency improvement with no user-facing API changes.