[ci-scan] Fix crossgen2_comparison.py Python 3.14 asyncio compatibility

[github-actions]

Summary

Fixes the RuntimeError: There is no current event loop in thread 'MainThread' crash in crossgen2_comparison.py on macOS Helix machines that have upgraded to Python 3.14.

Changes

Replaced the deprecated pattern:

loop = asyncio.get_event_loop()
loop.run_until_complete(coro)
loop.close()

with the modern equivalent:

asyncio.run(coro)

asyncio.run() was introduced in Python 3.7, respects the event loop policy (including the WindowsProactorEventLoopPolicy set in __init__), and is the recommended replacement for the removed implicit event loop behavior.

Motivation

Python 3.14 removed the deprecated implicit event loop creation in asyncio.get_event_loop() when called from the main thread with no running loop. This broke the crossgen2_comparison.py script on macOS arm64 Helix agents where Homebrew installed Python 3.14.

Affected pipeline: runtime-coreclr crossgen2 outerloop (def 134) — osx-arm64 leg

Fixes #129211

Testing

The fix is a straightforward API migration with identical semantics. The script's existing test coverage (crossgen2 comparison runs) validates correctness.

Note

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Generated by CI Outer-Loop Failure Scanner · ● 100.8M · ◷

[jkotas]

/azp run runtime-coreclr crossgen2 outerloop

[Copilot]

Pull request overview

Updates the crossgen2 comparison test script to use asyncio.run(...) instead of manually creating/closing an event loop, improving compatibility with newer Python asyncio behavior.

Changes:

• Replace asyncio.get_event_loop() + run_until_complete() + close() with asyncio.run(...) in AsyncSubprocessHelper.run_to_completion.
• Keep the script’s async orchestration model intact (still uses asyncio.gather over scheduled tasks).

[jkotas]

/azp run runtime-coreclr crossgen2 outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jkotas]

/ba-g known infra timeouts. Change affects crossgen outerloop only that's not covered by build analysis.