feat(connectors): add direct gateway calls

[verdie-g]

Summary

• add call_mcp_tool_async for stateless MCP tool calls through the connectors gateway
• add call_http_endpoint_async for relative httpx.Request objects, returning the raw httpx.Response
• reuse the SDK's configured async HTTP client and API authentication
• support named connector credentials on both methods

Behavior

• absolute HTTP request URLs and dot-segment traversal are rejected
• redirects and automatic retries are disabled for direct gateway calls
• caller-provided HTTP Authorization headers are preserved
• MCP results are converted to the existing ConnectorToolCallResponse model
• no optional MCP client dependency is required

Testing

• src/mistralai/extra/tests/test_connectors_gateway.py: 9 passed
• custom-code lint suite: mypy, pyright, and ruff passed
• extra test suite excluding one GC-order-sensitive test: 345 passed, 12 subtests passed
• the deselected GC cleanup test passes independently

[andreaonofrei01]

hello sir, if possible bring this to the monorepo client-python because everything in this repo gets overwritten(because of the move to the monorepo)

[verdie-g]

Closing in favor of moving this change to the monorepo, as requested in review.