Add E2E tests for malformed MCP protocol input

[jserv]

Summary

The E2E test suite lacks adversarial protocol cases. These are needed before implementing stricter protocol validation.

Coverage needed

• Missing jsonrpc field, non-"2.0" version, id as array/object.
• params as non-object (array/scalar/null).
• Empty method, method with trailing whitespace.
• Notification with unexpected id (warn, no response).
• Gate: ≥6 new malformed-input E2E scenarios.