Add comprehensive integration tests for proxy caching functionality

[Copilot]

This PR adds a complete integration test suite that validates the core caching behavior of proxy-kutti, proving that requests are correctly forwarded to origin servers on cache misses and served from cache on cache hits.

What's Added

Test Suite Structure

• test/real-proxy.test.js - Primary integration test validating actual cache behavior
• test/proxy-cache.test.js - Simplified cache logic test with mock implementation
• test/integration.test.js - Basic integration test framework
• test/README.md - Comprehensive documentation for the test suite

Test Scripts in package.json

"scripts": {
  "test": "node test/real-proxy.test.js",
  "test:cache": "node test/proxy-cache.test.js", 
  "test:integration": "node test/integration.test.js",
  "test:all": "npm run test && npm run test:cache && npm run test:integration"
}

What the Tests Prove

Cache Miss Behavior ✅

• Requests are forwarded to origin server when no cache exists
• Origin server receives and processes the request
• Response is cached in the correct filesystem structure: cache_dir/protocol/host:port/method/path.data
• Cache metadata files (.data.meta) are created with headers and request information

Cache Hit Behavior ✅

• Subsequent identical requests are served from cache without hitting origin
• Origin server request count remains unchanged for cached responses
• Cached responses match original responses exactly
• No additional network requests are made

Cache File Structure ✅

• Files follow the documented proxy-kutti structure
• Metadata includes status codes, headers, and original request details
• ETag and Last-Modified headers are properly preserved for cache validation

Example Test Output

🎉 All real proxy integration tests passed!
Total origin server requests: 4
Cache directory: /tmp/proxy-kutti-real-test

Cache directory structure:
/tmp/proxy-kutti-real-test/http/127.0.0.1:9006/GET/api/etag-test.data
/tmp/proxy-kutti-real-test/http/127.0.0.1:9006/GET/api/cache-miss-test.data.meta
...

Test Architecture

The tests use:

• Mock HTTP origin servers that track request counts
• Temporary cache directories to avoid polluting real cache
• Direct filesystem verification of cache file structure
• Request counting to definitively prove cache hits vs misses

This provides concrete proof that proxy-kutti's caching functionality works as designed, ensuring requests are only forwarded to origin servers when necessary and served efficiently from cache when possible.

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