fix: guard debug stack walk

[windsornguyen]

Summary

Guard uvloop’s debug stack capture on Python 3.14 by walking frames defensively instead of calling traceback.walk_stack() directly. This avoids AttributeError / segfaults when async generator finalization surfaces non‑frame objects (e.g. _asyncio.TaskStepMethWrapper) under debug mode.

Details

• Replaces traceback.walk_stack() with a simple f_back walk that only includes frames with f_lineno and f_code.
• Keeps debug stack capture intact while preventing crashes.

Repro (from #715)

Enable debug mode and run an async‑generator chain that finalizes under uvloop:

• PYTHONASYNCIODEBUG=1
• Python 3.14 + uvloop 0.22.1

See issue #715 for the minimal script and traceback.

Tests

• python3 -m pytest