Python: fix(core): workflow cancellation not propagating to active executors

[moonbox3]

Motivation and Context

When a workflow is cancelled via asyncio.create_task() + task.cancel(), executors that are mid-execution continue running to completion instead of being cancelled. This only affects executors after the first one (Step2+), because the CancelledError is raised in the polling loop but the iteration_task created via asyncio.create_task() is orphaned and keeps running.

Description

• Fixes Python: [Bug]: Workflow cancellation ignored by subsequent agents #3608
• Add unit test coverage

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/_workflows
_runner.py	220	32	85%	132–133, 173–176, 180, 221–223, 248–249, 251, 286–288, 312–316, 320, 355, 359, 361, 367, 375–378, 391, 431
TOTAL	16345	1982	87%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3823	221 💤	0 ❌	0 🔥	1m 12s ⏱️

[Copilot]

Pull request overview

This PR fixes a bug where workflow cancellation was not properly propagating to active executors. When a workflow was cancelled during the execution of the second or subsequent executor, the active executor would continue running to completion instead of being cancelled.

Changes:

• Added cancellation handling in the workflow runner's polling loop to cancel orphaned iteration tasks
• Added comprehensive test coverage for workflow cancellation behavior

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/packages/core/agent_framework/_workflows/_runner.py	Added try-except block to catch CancelledError in polling loop, cancel iteration_task, and properly clean it up before re-raising
python/packages/core/tests/workflow/test_runner.py	Added SlowExecutor test helper class and test_runner_cancellation_stops_active_executor test to verify cancellation propagates correctly