`cancelAndCompose` in Race3.kt eagerly throws `CancellationException`

In every other situation where `mergeSuppressed` is used, `CancellationException` is special-cased and not treated as "fatal", except here. Thus, if `first` was already cancelled, `second`'s exception, regardless of what it is, will be ignored. This isn't great since we really should be using `mergeSuppressed` in this case, especially if changes like #3924 make it through.

Start by reading Race3.kt and locate cancelAndCompose and the mergeSuppressed usage described in the issue. Verify the behavior when first is already cancelled and second has another exception, then confirm the relevant test suite covers the corrected exception handling.

Written by the indexing model from the issue text.