fix(mcp): complete oversized request responses

[opaopa6969]

Closes #99

Summary

• drain oversized request bodies before returning HTTP 413
• inspect max+1 bytes so chunked/unknown-length bodies are rejected
• add regressions for fixed-length and chunked requests, including response Content-Length

Testing

• git diff --check passed
• Maven test could not run because this environment has no Java runtime (mvn/mvnw unavailable without Java)