Add /v1/responses route scaffolding

[HXYerror]

Part of #1.

Goal

Expose POST /v1/responses (and POST /responses) as a new Hono route, mirroring the existing /v1/chat/completions and /v1/messages patterns. This issue is route + request validation only; the upstream call lives in a separate issue.

Current state

src/server.ts:17–31 registers exactly:

```
/, /chat/completions, /v1/chat/completions, /models, /v1/models,
/embeddings, /v1/embeddings, /usage, /token,
/v1/messages, /v1/messages/count_tokens
```

There is no /responses directory under src/routes/.

Tasks

• Create src/routes/responses/ mirroring the chat-completions/ layout
  • route.ts — Hono route with POST handler
  • types.ts — zod schema for the OpenAI Responses API request body (model, input, instructions, tools, tool_choice, temperature, top_p, max_output_tokens, reasoning, previous_response_id, store, include, stream, metadata, parallel_tool_calls, service_tier, truncation, user)
  • handler.ts — request → service → response wiring
• Mount in src/server.ts at both /responses and /v1/responses
• Plumb the rate-limit, manual-approval, and verbose-logging middleware that /chat/completions already uses
• Return 501 (or pass through to a stub upstream call) until the service client lands

Acceptance criteria

• POST /v1/responses returns a structured 4xx for malformed bodies (zod validation) instead of 500
• Hitting it with a valid body produces traffic visible under --verbose logging
• Existing /v1/chat/completions and /v1/messages regression tests still pass

File pointers

• src/server.ts:17–31
• src/routes/chat-completions/ (reference layout)
• src/routes/messages/route.ts:10 (reference for nested route under /v1)

[HXYerror]

Implemented in commits a36fa09, d417a7c, 977a30f on master. Route mounted at /responses and /v1/responses; full OpenAI Responses API types; stub 501 handler with try/catch; 3 route tests. Service client wiring follows in #4.