mcp_transcoder: handle responses with trailers.

[guoyilin42]

Commit Message: mcp_transcoder: handle responses with trailers.
Additional Description:
This PR adds support for transcoding upstream responses with HTTP trailers. The McpJsonRestBridgeFilter::encodeTrailers method is now implemented to properly finalize the response stream:

• Streaming JSON: If a streaming JSON suffix is configured, the suffix is appended upon the arrival of trailers. If the body chunk was empty, both the streaming JSON prefix and suffix are injected to form a valid empty response envelope.
• Buffered (Non-Streaming) Body: When trailers arrive, the filter transcodes the response headers using encodeJsonRpcData and flushes the buffered transcoded response body downstream.

Risk Level: Low
Testing: Unit and integ tests
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A

[guoyilin42]

#44146

@paulhong01