🧪 Add extensive unit tests for ResponseCache edge cases

[maattyi]

🎯 What: The testing gap in edge cases for the ResponseCache object was addressed by introducing the unit tests in tests/test_response_cache.py.

📊 Coverage: Scenarios covered include:

• Base states / default values of the Cache objects (hits, misses, _size)
• Tracking logic of cache hits/misses
• TTL eviction bounds on retrieval limits
• Cache Eviction rules (ensuring oldest element removal bounds are accurate)
• Payloads replacements to ensure correct memory tracking calculations
• Limits rejections for overly large values > 25% and empty responses
• Parse TTL logic heuristics including max-age rules, no-store, Content-Type heuristics and static file resolution extensions.

✨ Result: Test coverage significantly improves as all functions and constraints relating to caching logic inside src/proxy_server.py:ResponseCache have coverage, leading to easier refactoring abilities without introducing unhandled regression errors.

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PR created automatically by Jules for task 12747109781506660356 started by @maattyi

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