[test] Add tests for server.rate_limit, server.middleware_auth, and server.response_writer

[github-actions]

Test Coverage Improvement

Functions Analyzed

• Package: internal/server
• Files: rate_limit.go, middleware_auth.go, response_writer.go
• Previous Coverage: 0% (no test files existed)
• Complexity: Medium–High

Why These Functions?

Three files in internal/server had zero test coverage despite containing meaningful business logic:

• rate_limit.go — Detects GitHub API rate-limit errors from tool results, including text pattern matching and reset-time extraction
• middleware_auth.go — API key authentication middleware implementing spec section 7.1, including malformed header detection
• response_writer.go — HTTP response wrapper that captures body and status code for logging

Tests Added

rate_limit_test.go

• ✅ isRateLimitText — all known rate-limit patterns (case-insensitive), negative cases, partial matches
• ✅ extractRateLimitErrorText — MCP content extraction, non-map inputs, empty content fallback
• ✅ isRateLimitToolResult — error vs success results, reset-time parsing, secondary rate limits, too-many-requests

middleware_auth_test.go

• ✅ authMiddleware — valid key, missing header (401), wrong key (401), malformed header with null byte (400)
• ✅ applyIfConfigured — middleware applied when key non-empty, passthrough when key empty
• ✅ applyAuthIfConfigured — end-to-end with and without API key

response_writer_test.go

• ✅ newResponseWriter — initial state (zero status, empty body)
• ✅ WriteHeader — status code captured in wrapper and underlying recorder
• ✅ Write — body captured in buffer and passed through to underlying writer
• ✅ Multiple writes — body accumulates correctly
• ✅ Implicit 200 on first write
• ✅ Header passthrough
• ✅ Interface compliance (http.ResponseWriter)

Coverage Report

Before: 0% (no test files)
After:  ~90%+ coverage for these three files

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