[JS/TS/Python] Fix Async.StartChild with timeout always timing out

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Closes #4481

Summary

• Root cause: startChild used Promise.all (join) to race the computation against a throwAfter timer. Since throwAfter only ever rejects (after the timeout elapses) and never resolves, Promise.all could never resolve — it always waited for both branches, then rejected with TimeoutException regardless of how fast the computation finished.

• Fix: Use race semantics instead. The computation is started as a Promise immediately, then a new racing Promise wraps both the computation and a plain setTimeout reject. Whichever settles first wins — computation success/failure propagates through; timeout fires TimeoutException only if the computation hasn't settled yet.

• Cleanup: Remove the now-unused throwAfter and parallel2 helper functions.

• Test: Added a regression test "Async.StartChild with timeout completes when computation finishes before timeout" that verifies a 10 ms computation succeeds under a 1000 ms timeout.

Changes

• src/fable-library-ts/Async.ts — fix startChild, remove dead helpers
• tests/Js/Main/AsyncTests.fs — add regression test (see [Investigate] Async.StartChild with timeout always times out in JavaScript #4481)
• src/Fable.Cli/CHANGELOG.md and src/Fable.Compiler/CHANGELOG.md — update ## Unreleased

Trade-offs

The new implementation uses a plain setTimeout for the timeout (no cancel-listener integration). The previous throwAfter had cancel-listener support, but since startChild already starts the computation in a hot Promise (cancellation of the outer token does not cancel the child promise anyway), this is not a regression. The semantics match .NET's Async.StartChild: once started, the child runs to completion unless it times out.

Note

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Python Type Checking Results (Pyright)

Metric	Value
Total errors	18
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_hash_set.py	3	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	1	Excluded