feat: Enable SSE transport for mcp service and update test fixtures

Changes:
- Remove streamable-http transport override from mcp service in docker-compose.yml
- Service now uses CLI default SSE transport on /sse endpoint
- Add create_mcp_client_session_sse() helper for SSE connections
- Update nc_mcp_client fixture to use SSE transport
- Fix unpacking for SSE client (yields 2 values vs 3 for streamable-http)

Testing:
- All 4 smoke tests pass with SSE transport
- 32/34 affected tests pass (2 skipped for vector sync)
- OAuth services remain on streamable-http (unchanged)

Note: SSE transport is being deprecated in favor of streamable-http.
This enables minimal validation testing before deprecation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docker-compose.yml

@@ -69,7 +69,6 @@ services:
 
   mcp:
     build: .
-    command: ["--transport", "streamable-http"]
     restart: always
     depends_on:
       app: