Python: avoid MCP header injection on redirects

[he-yufeng]

Fixes #5820.

Summary

• only inject MCP header_provider headers for requests that stay on the configured MCP server origin
• preserve same-origin header injection for the existing streamable HTTP client path
• add a real httpx redirect regression test that verifies Authorization is not re-applied to a cross-origin redirect target

To verify

• python -m py_compile packages\\core\\agent_framework\\_mcp.py packages\\core\\tests\\core\\test_mcp.py
• uv run ruff check packages\\core\\agent_framework\\_mcp.py packages\\core\\tests\\core\\test_mcp.py
• uv run pytest packages\\core\\tests\\core\\test_mcp.py -q -k "header_provider_with_httpx_event_hook or header_provider_skips_cross_origin_redirect or header_provider_with_user_httpx_client" --basetemp .tmp\\pytest -p no:cacheprovider
• git diff --check

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