feat(adapters): add mcp_tool_interceptor() to AxonFlowLangGraphAdapter

[gzak]

Summary

Closes #107.

Adds mcp_tool_interceptor() factory method to AxonFlowLangGraphAdapter, returning an async callable ready to pass directly to MultiServerMCPClient:

mcp_client = MultiServerMCPClient(
    {"my-server": {"url": "...", "transport": "http"}},
    tool_interceptors=[adapter.mcp_tool_interceptor()],
)

The interceptor enforces AxonFlow input and output policies around every MCP tool call: mcp_check_input → handler() → mcp_check_output.

Design decisions

Redacted output passthrough — when mcp_check_output returns redacted_data, the interceptor substitutes it for the original result rather than discarding it silently:

if output_check.redacted_data is not None:
    return output_check.redacted_data
return result

Pluggable connector type derivation — accepts an optional connector_type_fn callable to override the default "{server_name}.{tool_name}" mapping:

adapter.mcp_tool_interceptor(
    connector_type_fn=lambda req: req.server_name
)

No new runtime dependencies

MCPToolCallRequest from langchain-mcp-adapters is only used for type hints under TYPE_CHECKING. The interceptor duck-types request.server_name, request.name, and request.args at runtime.

Test plan

• 12 new unit tests in tests/test_langgraph_adapter.py covering: allowed call, connector type derivation, input blocked, output blocked, redacted output passthrough, custom connector_type_fn, factory independence
• Full existing test suite passes (592 tests)