Fix OpenTelemetry span status for failed tasks (#136)

[chrisguidry]

Summary

• Fix OpenTelemetry spans not being marked as failing when tasks fail and are retried
• Set span status to ERROR when tasks fail, regardless of retries
• Set span status to OK when tasks succeed
• Use record_exception() for proper exception tracking
• Capture span directly from context manager instead of get_current_span()

Closes #136

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[github-actions]

📚 Documentation has been built for this PR!

You can download the documentation directly here:
https://github.com/chrisguidry/docket/actions/runs/15768172105/artifacts/3366520683

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (ca58e02) to head (3266af0).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #137   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines         3293      3354   +61     
  Branches       186       189    +3     
=========================================
+ Hits          3293      3354   +61     

Flag	Coverage Δ
python-3.12	100.00% <100.00%> (ø)
python-3.13	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/docket/worker.py	100.00% <100.00%> (ø)
tests/test_instrumentation.py	100.00% <100.00%> (ø)

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