feat: add fastapi-mcp-client extension (fixes #80)

[emberb170d]

Problem

MCP (Model Context Protocol) is a reusable capability for chat and multi-agent apps. CPA needs a focused MCP client extension that can be composed with FastAPI AI apps.

Solution

Create extensions/fastapi-mcp-client compatible with fastapi-backend.

Changes Made

• Added extensions/fastapi-mcp-client/ directory with complete extension structure
• Created app/core/mcp_client.py with typed MCP client implementation
• Added documentation in template/docs/MCP_CLIENT_GUIDE.md
• Included test suite in tests/
• Updated extension metadata in pyproject.toml

Key Features

• MCP server configuration via mcp_servers.yaml or env-driven equivalent
• Typed tool registry/service for discovering and using MCP tools
• Environment-based configuration support
• FastAPI integration with TestClient-verified endpoints

This extension addresses issue #80 by providing a reusable MCP client capability that can be composed with FastAPI AI applications without duplicating MCP logic.

Fixes #80

[coderabbitai]

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