fix: proper propagation of context cancellation errors

[sashaaro]

Previously, context cancellation errors were converted into plain strings "...: context canceled", which caused:

errors.Is(err, context.Canceled)

to always return false;
inability to reliably distinguish “normal” context cancellation from real errors.

Now context cancellation is propagated as-is, and client code can correctly check the cause using errors.Is or context.Cause.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.96%. Comparing base (476693b) to head (badfd43).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #213      +/-   ##
==========================================
- Coverage   45.23%   44.96%   -0.27%     
==========================================
  Files          29       29              
  Lines        2352     2355       +3     
==========================================
- Hits         1064     1059       -5     
- Misses       1238     1247       +9     
+ Partials       50       49       -1     

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