[repo-assist] tests: add coverage for side-effect re-execution with independent cancellation tokens

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Summary

Adds a SideEffects module to TaskSeq.CancellationToken.Tests.fs covering a gap identified while reviewing test coverage: what happens to a taskSeq's side effects when it is enumerated multiple times with independent CancellationTokens (including one that was cancelled and disposed). This complements the existing cancellation-propagation tests in the same file, which only check that cancellation throws — not whether re-enumeration state and side effects behave correctly afterwards.

Two new tests:

• Cancelling and disposing one enumerator does not corrupt or block a fresh enumeration of the same taskSeq with a new, non-cancelled token; the body re-runs from scratch and side effects accumulate as expected.
• Re-iterating with CancellationToken.None after a first pass with an explicit token re-executes the generator body independently (each GetAsyncEnumerator call is a fresh run).

No production code was changed — this is a test-only addition.

Test Status

• dotnet fantomas . --check: passes
• dotnet test -c Release --filter FullyQualifiedName~CancellationToken: 19 passed, 0 failed
• Full build succeeds

AI disclosure

This PR was created by Repo Assist, an automated AI assistant, as part of routine testing improvements (Task 5: Coding Improvements / test coverage). A human maintainer should review before merging.

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