Fix #105: don't crash collecting actor tracebacks for finished coroutines

[wbarnha]

What

Agent.actor_tracebacks() iterated every actor calling actor.traceback(), which delegates to mode's coroutine / async-generator stack formatter. A coroutine or async generator that has finished naturally has no stack frame (cr_frame / ag_frame is None), and mode raises RuntimeError('cannot find stack of coroutine') when asked to format it.

Traceback collection runs inside Consumer.wait_empty around shutdown and rebalances, purely to log agent state. Because the old code built the list in one comprehension, a single finished actor aborted the whole collection — and with it a clean shutdown/rebalance.

Fixes #105.

How

Collect tracebacks one actor at a time and catch RuntimeError per actor, substituting a short placeholder (Could not extract traceback for actor <actor>: <exc>) instead of letting it propagate. This mirrors the workaround several users adopted via a custom Agent subclass, but folds it into faust itself so no subclass is required.

Test

Added test_actor_tracebacks and test_actor_tracebacks__missing_stack in tests/unit/agents/test_agent.py: the first confirms normal collection, the second confirms a RuntimeError('cannot find stack of coroutine') from one actor is swallowed into a placeholder while a sibling actor's traceback is still returned.

🤖 Generated with Claude Code

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.16%. Comparing base (3073eb9) to head (3efadc8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #698      +/-   ##
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+ Coverage   94.14%   94.16%   +0.02%     
==========================================
  Files         104      104              
  Lines       11136    11142       +6     
  Branches     1201     1202       +1     
==========================================
+ Hits        10484    10492       +8     
+ Misses        551      549       -2     
  Partials      101      101              

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