feat(mcp): replace hand-rolled HTTP with SDK streamable_http_client

[Hundao]

Description

Replace the hand-rolled httpx-based HTTP implementation in MCPClient with the MCP SDK's streamable_http_client, making the HTTP transport spec-compliant and unifying both STDIO and HTTP code paths.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #3152 (Step 1 — Transport Protocol)

Changes Made

• Replace httpx.Client + hand-rolled JSON-RPC with MCP SDK's streamable_http_client + ClientSession
• Unify STDIO and HTTP transports to share the same session-based tool listing, invocation, and cleanup logic
• Auto-append /mcp to URLs without a path (matches FastMCP's default Streamable HTTP endpoint)
• Remove import httpx top-level dependency
• Properly close httpx.AsyncClient in _cleanup_async() (SDK does not auto-close a provided client)
• Add 15 unit tests (test_mcp_client.py) covering URL handling, validation, unified interface, cleanup, and config
• Add 7 integration tests (test_mcp_client_integration.py) with an in-process FastMCP server over real HTTP

Testing

• Unit tests pass (cd core && pytest tests/test_mcp_client.py) — 15/15
• Integration tests pass (cd core && pytest tests/test_mcp_client_integration.py) — 7/7
• Lint passes (ruff check)
• Format passes (ruff format --check)
• Manual E2E testing with two example agents:
  • web_research_agent (STDIO transport) — searched topic + sent email via hive-tools MCP
  • place_finder_agent (HTTP transport) — searched places via Google Maps MCP at https://mapstools.googleapis.com/mcp

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)

E2E: web_research_agent (STDIO transport)

E2E: place_finder_agent (HTTP transport → Google Maps MCP)

[TimothyZhang7]

Testing