Rewrite frame_cb tests to avoid coroutine_handle UB

[mvandeberg]

The original tests used from_address to create a coroutine_handle from a fake frame_cb struct. This violates [coroutine.handle.export.import] and LLVM exploits the UB at -O1+, compiling the tests to unreachable.

Tests 1-3: call frame_cb function pointers directly instead of routing through coroutine_handle.

Tests 4-5: replace fake frame + from_address with real task
coroutines that co_await custom IoAwaitables, using run_blocking (sync) and run_async + thread_pool (async).

[coderabbitai]

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[cppalliance-bot]

An automated preview of the documentation is available at https://240.capy.prtest3.cppalliance.org/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-03-21 02:06:46 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.37%. Comparing base (fcd2854) to head (cae9c9f).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #240      +/-   ##
===========================================
- Coverage    92.38%   92.37%   -0.02%     
===========================================
  Files          168      168              
  Lines         9327     9327              
===========================================
- Hits          8617     8616       -1     
- Misses         710      711       +1     

Flag	Coverage Δ
linux	92.35% <ø> (-0.03%)	⬇️

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see 1 file with indirect coverage changes

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