fix(cdk-assets-lib): pLimit.dispose() doesn't stop new jobs from starting

[Adityaj0]

Closes #1876

Reason for this change

pLimit()'s dispose() method is documented and tested to stop new jobs from starting, but dispatch() (called every time a new task is submitted) never checked the stopped flag - only resumeNext() (which runs when an already-active job finishes) did. So a task submitted after dispose() would start immediately whenever a concurrency slot happened to be free at submission time, silently violating dispose()'s contract.

Description of changes

• dispatch() now returns immediately if stopped is true, so no new job starts after dispose().
• ret() now rejects immediately with 'Task has been cancelled' if a task is submitted after dispose(), so callers don't hang waiting on a promise that would otherwise never settle (queued-but-never-dispatched).
• Added a regression test to test/private/p-limit.test.ts that fails before the fix (job runs despite dispose()) and passes after.

Description of how you validated changes

Ran the full @aws-cdk/cdk-assets-lib test suite locally - all 104 tests across 14 suites pass, including the new regression test and the existing p-limit.test.ts tests.

Test Suites: 14 passed, 14 total
Tests:       104 passed, 104 total

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.10%. Comparing base (7ff50e7) to head (49611cf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1877   +/-   ##
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  Coverage   91.10%   91.10%           
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  Files          80       80           
  Lines       12205    12205           
  Branches     1742     1742           
=======================================
  Hits        11119    11119           
  Misses       1050     1050           
  Partials       36       36           

Flag	Coverage Δ
suite.unit	91.10% <ø> (ø)

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