Python: Fix MCP header provider across transport tasks

[kiwigitops]

Fixes #4808.

MCPStreamableHTTPTool currently stores per-call header_provider output in a ContextVar that the httpx request hook reads later. With the streamable HTTP transport, outbound requests can be sent from a transport task that was created before call_tool() set that context, so the hook can see an empty value and skip the dynamic headers.

This keeps the active headers on the tool instance for the duration of the tool call, so the existing request hook reads the current header snapshot even when the transport sends from another task. The previous value is restored after the call completes.

Tested:

• python -m pytest python/packages/core/tests/core/test_mcp.py -k header_provider -q
• python -m ruff check python/packages/core/agent_framework/_mcp.py python/packages/core/tests/core/test_mcp.py
• git diff --check

[kiwigitops]

Closing this because #6145 has landed on main and covers the same header propagation problem with the newer ContextVar approach. Current main now stores per-call MCP headers in _mcp_call_headers and includes coverage for both transport-task request hooks and invoke-context runtime kwargs, so keeping this older conflicting branch open would just duplicate that fix.