Commit Message: mcp_transcoder: add sse server response support.
Additional Description:
This PR introduces Server-Sent Events (SSE) server response support to the MCP JSON REST bridge filter. When the upstream server responds with a text/event-stream content type, the filter will now parse the stream and wrap each extracted complete event payload into a distinct text content item within the JSON-RPC content array on the fly.

Key Changes:

• Added SseResponseExtractor: Introduced sse_response_extractor.cc and its header to process incoming chunks of an SSE stream and return completed event payloads.
• Updated McpJsonRestBridgeFilter: Modified the encoding path to check for the text/event-stream content type. It now delegates chunk processing to the new SSE extractor and formats the results into the JSON-RPC streaming envelope.
• Testing: Added unit tests in sse_response_extractor_test.cc to verify the handling of single/multiple events, multiline data, incomplete chunked events, and CRLF line endings. Also added ToolsCallSseStreamingTranscoding to integration tests.

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

Request variables

Key	Value
ref	10f0fa3
sha	73132c3
pr	45374
base-sha	af5b6f3
actor	@guoyilin42
message	mcp_transcoder: add sse server response support....
started	1780444809.176085
target-branch	main
trusted	false

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