fix(gateway): reject invalid and oversized JSON bodies

[YOMXXX]

Summary

• return HTTP 400 for malformed JSON request bodies instead of treating them as server errors
• cap gateway JSON request bodies at 8 MiB using both Content-Length and streamed byte counting
• add gateway regression tests for malformed and oversized request bodies

Test plan

• npx vitest run src/gateway
• npm test
• npm run build

Notes

• npm run test:cc-plugin -- --run currently fails because package.json has no test:cc-plugin script.