…call_tool requests (#5158)

Motivation and Context

MCP servers can accept a _meta field alongside tool arguments in call_tool requests, enabling clients to pass request-level metadata such as correlation IDs, user locale, session info, and other context that servers may use for debugging, distributed tracing, or request-specific behavior.

Both OpenAI Agents SDK (PR #2375) and LangChain MCP Adapters (PR #468) have recently added _meta support, making this an expected capability across MCP client frameworks.

Description

Propagate user-supplied _meta from function_invocation_kwargs to MCP call_tool requests. The change is minimal (~4 lines in MCPTool.call_tool()):

1. Extract _meta from incoming **kwargs via kwargs.pop("_meta", None) before filtering
2. Add "_meta" to the existing filtered-kwargs exclusion set so it never leaks into tool arguments
3. Pass the extracted dict to _inject_otel_into_mcp_meta(user_meta) instead of _inject_otel_into_mcp_meta() — user keys take precedence, OTel keys fill in non-conflicting slots

This uses the established function_invocation_kwargs mechanism — the same per-request injection pattern already used across 6+ samples for API keys, user IDs, and session metadata:

response = await agent.run(
    "Search for Python repos",
    function_invocation_kwargs={
        "_meta": {
            "correlation_id": "req-456",
            "session_id": "abc-123",
            "locale": "fr-FR",
        },
    },
)

No new public types, parameters, or constructor changes. Fully backward compatible — when _meta is not provided, behavior is identical to before.

Files changed:

• python/packages/core/agent_framework/_mcp.py — extract and forward _meta in call_tool()
• python/packages/core/tests/core/test_mcp.py — 9 new tests covering all edge cases
• docs/specs/002-mcp-meta-support.md — feature spec

Test coverage:

• _meta forwarded as protocol-level meta, excluded from tool arguments
• User meta merged with OTel trace context; user keys win on conflicts
• Backward compatibility when _meta is absent
• Caller's dict not mutated by OTel injection
• Empty _meta={} treated as no user meta
• End-to-end flow through FunctionTool wrapper (as created by load_tools())
• MCPStreamableHTTPTool override forwards _meta to super().call_tool()
• Combined with header_provider without interference

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.