[Bug] Make _self_call_placeholder_ thread-safe

[hudson-ai]

Puts _self_call_placeholder_ in threading.local() to prevent other threads from tricking the decorator into thinking we're in a recursive call.

Small note: the thread-local storage is in the wrapped function's closure, not an attribute on the function itself.

Tests would be nice, but annoyingly I can only stochastically reproduce the issue as it is due to a race condition. Increasing the number of threads and/or jobs might make the bug occur with higher probability though -- happy to add @mmoskal's repro if we're all okay with a slightly stochastic test...

Closes #1031

[hudson-ai]

Ping @mmoskal (for some reason, I can't mark you as a reviewer... not in the org?)

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.47%. Comparing base (edd8b87) to head (e406a70).
Report is 1 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1034      +/-   ##
==========================================
- Coverage   70.16%   61.47%   -8.69%     
==========================================
  Files          62       62              
  Lines        4471     4473       +2     
==========================================
- Hits         3137     2750     -387     
- Misses       1334     1723     +389     

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