fix: task cancellation from non-main thread

[rmshaffer]

Issue #, if available:
fixes #1305

Description of changes:
When cancelling a quantum task, there is no reason to create a future just to cancel it; and in fact this doesn't work on non-main threads (or even on main threads in Python 3.14 - although this is not officially supported yet) where there is no event loop.

Testing done:

Merge Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a1b1866) to head (b4e2a40).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1306   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          171       171           
  Lines        11404     11403    -1     
  Branches      1481      1481           
=========================================
- Hits         11404     11403    -1     

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