mcp_transcoder: increase integ test coverage.

[guoyilin42]

Commit Message: mcp_transcoder: increase integ test coverage.
Additional Description:

The current Envoy test suite primarily focuses on happy-path scenarios for initialize, notifications/initialized, tools/list, and tools/call. It lacks coverage in the following key areas:

• JSON-RPC Protocol & HTTP Request Validation: Handling of malformed JSON, missing required fields (method, id), unsupported methods, and invalid HTTP verbs.
• Method-Specific Validation Errors: Handling of missing parameters for specific methods (e.g., protocolVersion in initialize, missing/unknown tools in tools/call).
• Upstream Error Handling: Translating upstream backend HTTP/gRPC errors into the appropriate MCP JSON-RPC error format (isError: true).

Risk Level: Low
Testing: Integ tests.
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A

[guoyilin42]

#44146

cc @paulhong01

[tyxia]

/coverage

[repokitteh-read-only]

Coverage for this Pull Request will be rendered here:

https://storage.googleapis.com/envoy-cncf-pr/46049/coverage/index.html

For comparison, current coverage on main branch is here:

https://storage.googleapis.com/envoy-cncf-postsubmit/main/coverage/index.html

The coverage results are (re-)rendered each time the CI Envoy/Checks (coverage) job completes.

🐱

Caused by: a #46049 (comment) was created by @tyxia.

see: more, trace.