feat(server): request-body size cap middleware (closes #239)

[bokelley]

Summary

Closes #239 — the Medium security finding deferred from PR #238. Every tool call runs Pydantic `model_validate` unconditionally now that typed-dispatch is live; without an input-size bound, adversarial callers can exhaust validation CPU/memory with arbitrarily large JSON.

Adds an ASGI middleware installed at the `serve()` bind point — caps HTTP request bodies at 10 MB by default, rejects oversized bodies with HTTP 413 before FastMCP or a2a-sdk parse a byte.

Two enforcement layers

• Content-Length fast-fail. Advertised body over the cap → reject without reading.
• Streaming accounting. No Content-Length (chunked transfers) → middleware buffers + counts, 413s the moment the total crosses the cap.

GET / HEAD / OPTIONS bypass (no request body). Lifespan + WebSocket scopes pass through untouched.

API

```python

Default: 10 MB

serve(MyAgent())

Sellers with legitimately large creative asset payloads:

serve(MyAgent(), max_request_size=50 * 1024 * 1024)

Public-facing deployments that only accept small payloads:

serve(MyAgent(), max_request_size=256 * 1024)

Escape hatch — enforce elsewhere (reverse proxy / WAF):

serve(MyAgent(), max_request_size=0)
```

Covers both transports (MCP streamable-http + SSE, A2A). stdio skips since there's no HTTP body to police.

Test plan

• 11 ASGI-level tests, all passing
• Content-Length reject (over cap), accept (at cap), accept (under cap)
• Malformed Content-Length falls through to streaming check
• Chunked body exceeding cap mid-stream rejects; inner app never invoked
• Chunked body under cap passes through verbatim
• GET/HEAD/OPTIONS bypass
• Lifespan scope passes through untouched
• `max_request_size=0` opt-out returns app unwrapped
• Explicit int overrides default
• Full suite: 1876/1876 pass
• mypy clean (673 source files)

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