[repo-assist] Add test coverage for tryFirst, firstOrDefault, zipWithParallel, combineLatestWithAsync, toObservable

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Summary

During routine coverage review, I identified several public AsyncSeq API functions with no existing test coverage: tryFirst, firstOrDefault, zipWithParallel, combineLatestWithAsync, and toObservable. This PR adds tests for each, covering both typical and edge-case (empty sequence) behavior. No production code was changed.

What was added

• tryFirst: Some/None for non-empty/empty sequences.
• firstOrDefault: default value vs. first element.
• zipWithParallel: combining two sequences, and stopping at the shorter one.
• combineLatestWithAsync: combine-latest semantics (seed + subsequent updates) and empty-source short-circuiting.
• toObservable: verifying OnNext/OnCompleted delivery for both non-empty and empty sequences via a minimal IObserver test double.

Trade-offs

• The toObservable tests use a ManualResetEventSlim with a timeout to wait for async completion since the observable bridges to a background async computation; this is consistent with how async-to-sync bridging is tested elsewhere but does add a small amount of wall-clock time to the test run.

Test Status

✅ dotnet test -c Release — 486/486 tests passed (476 pre-existing + 10 new), 0 failures. Pre-existing NU1605 (package downgrade) and FS9999 (groupBy consumption) warnings are unrelated to this change and present on main.

As always, please review before merging — happy to adjust test style or add further edge cases if useful!

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