Add comprehensive tests across all packages

[Copilot]

The handler package had zero test coverage, and other packages were missing important edge cases.

New: handler_test.go (~25 tests)

• ResponseCache: set/get, miss, expiration, clear, overwrite
• Path matching: extractPathParams, matchPath with single/multiple params, exact match, mismatches
• HandleRequest: GET, POST, DELETE, OPTIONS, 404, path param replacement, static file serving, CORS headers, response caching, cache invalidation
• getJSONResponse: param substitution, file not found, invalid JSON fallback

Extended: config_test.go (+12 tests)

• Default values (port→3000, logLevel→info, logFormat→text)
• Invalid JSON, file not found, reload with invalid config preserves original
• Thread-safe getters (GetEndpoints, GetPort, GetHost, GetLogConfig)
• Multiple HTTP methods on same path, empty jsonPath

Extended: logger_test.go (+11 tests)

• Level filtering (debug/info suppressed at warn level)
• File output with nested directory creation
• Variadic log methods (with/without data), JSON format output
• AccessLog with non-JSON content type, GET requests

Extended: middleware_test.go (+6 tests)

• CORS across all HTTP methods, recovery without panic
• Empty middleware chain, chain execution order verification
• Logger middleware with various status codes

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.