feat #262: MCP Langchain Agent

[vk-playground]

Implements the files covered by README for the MCP Langchain Agent:

• /health, /ready, /list_tools
• /v1/chat/completions (+ streaming)
• /a2a (JSON-RPC)
• LangChain agent, MCP client, models, config, start script, requirements
  No additional changes beyond README scope. Refs [Feature Request]: Sample Agent - LangChain Integration (OpenAI & A2A Endpoints) #262.

[MohanLaksh]

make serve - FAIL 2025-08-18 06:00:10,319 - mcpgateway.services.gateway_service - WARNING - Gateway mcp-server failed health check 1 time(s).

UI shows blank page in resources, prompts, servers and roots

make test - all pass - (76%, === 1719 passed, 11 skipped, 482 warnings in 60.93s (0:01:00) ===)
make autoflake isort black flake8 - pass - no errors

make pylint - FAIL- Your code has been rated at 9.96/10

make smoketest - pass - ✅ Smoketest passed!
make doctest - all pass - (50%, 477 passed, 6 skipped, 54 warnings in 13.83s)

[crivetimihai]

Could you merge this into main instead please? This IBM:resolve-ticket at the moment - which won't trigger CI/CD.

Let me know if you wish to merge into resolve-ticket first.

[vk-playground]

will merge into main instead