Python: fix(core): handle anyio cancel scope errors during MCP connection cleanup

[moonbox3]

Motivation and Context

When using multiple MCP tools with AG-UI streaming, the second MCP tool call would fail with:

RuntimeError: Attempted to exit cancel scope in a different task than it was entered in

This occurred because:

1. The AsyncExitStack in MCPTool stores context managers from the MCP library's streamable_http_client
2. The MCP library uses anyio.create_task_group() internally
3. When connect(reset=True) or close() is called from within an AG-UI streaming loop (a different async task than where the connection was established), anyio detects a task boundary violation

To fix the issue, this PR adds a _safe_close_exit_stack() helper method that:

• Catches anyio cancel scope RuntimeError variants and logs a warning instead of crashing
• Catches CancelledError that can occur during cleanup
• Allows garbage collection to clean up orphaned resources
• Replaces all direct _exit_stack.aclose() calls

Description

• Fixes Python: [AG-UI] Failure when connecting multiple MCP tools #3238
• Fixes Python: MCP stdio shutdown raises anyio cancel-scope error in agent_framework MCPStdioTool #2846
• Tested an AG-UI application with multiple MCP tools. No errors are thrown.

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.

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_mcp.py	388	57	85%	112, 174, 183, 246, 254, 275, 365, 432, 467, 469, 473–474, 476–477, 480–481, 484–485, 531, 546, 564, 605, 711, 724–729, 751, 766, 769–771, 786–787, 793–795, 814, 823, 826–828, 843–844, 848–852, 869–873, 1013
TOTAL	17437	2702	84%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3175	213 💤	0 ❌	0 🔥	1m 3s ⏱️

[Copilot]

Pull request overview

This PR fixes a critical bug where using multiple MCP tools with AG-UI streaming would fail with a RuntimeError when attempting to close cancel scopes across task boundaries. The fix introduces a new _safe_close_exit_stack() helper method that gracefully handles anyio cancel scope errors during connection cleanup.

Changes:

• Added _safe_close_exit_stack() method to handle cross-task cancel scope errors
• Replaced all direct _exit_stack.aclose() calls with the safe wrapper
• Added comprehensive tests for the new error handling behavior

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
python/packages/core/agent_framework/_mcp.py	Implements new _safe_close_exit_stack() method with error handling for anyio cancel scope errors and updates all call sites to use the safe wrapper
python/packages/core/tests/core/test_mcp.py	Adds three new test cases covering the main scenario, alternate error messages, and re-raising unrelated errors