Add tests for the /mcp body cap and complete the task file.
- oversized POST /mcp with declared Content-Length > 8 MiB -> 413
before any body read
- oversized chunked POST /mcp -> 413 (counting-stream cut mid-body;
rmcp maps body errors to 500, so the middleware sources the status)
- normal-size initialize round-trip unchanged
- task file: status completed, Summary filled
Verified: cargo test (219), cargo test --features mcp --lib (257),
cargo test --all-features (269 + integration), clippy default and
--all-features (-D warnings), cargo fmt --check.