[Repo Assist] test: expand choose/chooseAsync and filter/filterAsync test coverage

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Summary

Expands test coverage for TaskSeq.choose, TaskSeq.chooseAsync, TaskSeq.filter, and TaskSeq.filterAsync. These functions had only 7 tests each — this PR adds 8 new tests for choose/chooseAsync and 5 new tests for filter/filterAsync.

New tests

TaskSeq.Choose.Tests.fs

• All Some — chooser always returns Some passes all 10 elements through (sync + async)
• All None — chooser always returns None yields empty sequence (sync + async)
• Singleton with Some chooser — verifies correct result from a length-1 source
• Singleton with None chooser — verifies empty result from a length-1 source
• Type-changing chooser — int -> string option, exercises choose as a combined map+filter
• Evaluation count — each source element is visited exactly once even when some are discarded (sync + async)

TaskSeq.Filter.Tests.fs

• Keep all — predicate always true preserves all elements (sync + async via filter / filterAsync)
• Remove all — predicate always false yields empty (sync + async)
• Evaluation count — each source element is visited exactly once (sync)

Test Status

Build: ✅ succeeded (dotnet build src/FSharp.Control.TaskSeq.sln -c Release)
Tests: ✅ 169 passed, 0 failed (dotnet test --filter "TaskSeq.Tests.Choose|TaskSeq.Tests.Filter")

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