refactor: Modularize proxy and add tool_use support

- Split monolithic messages.js (223 lines) into 5 focused modules
- Add full tool_use/tool_result round-trip translation
- Add x-api-key header authentication (Anthropic SDK default)
- Fix SSE Content-Type via Hono streamSSE helper
- Fix streaming usage tracking with stream_options.include_usage
- Add stop_reason mapping (end_turn, max_tokens, tool_use, stop_sequence)
- Forward stop_sequences to OpenAI stop parameter
- Handle system message as string or array of content blocks
- Use timing-safe XOR comparison for auth tokens
- Cache OpenAI client and model map across requests
- Sanitize error responses to prevent upstream detail leakage
- Use crypto.randomUUID() for unique message IDs
- Remove non-existent build/dev commands from vercel.json

vercel.json

@@ -1,6 +1,4 @@
 {
-  "buildCommand": "npm run build",
-  "devCommand": "npm run dev",
   "installCommand": "npm install",
   "framework": null,
   "rewrites": [