Bug: Signal channel draining

[happylinks]

Added a test that shows the stuck workflow behavior from a rust workflow that listens to a sigchan. Discussion in slack: https://temporalio.slack.com/archives/C08G723SFNZ/p1747254628041319

What was changed

Added a test that gets stuck.

Why?

To show the stuck behavior and to try to get to a working version. Will continue investigating this.

Checklist

1. How was this tested:

RUST_LOG=debug,h2=info,tower=info cargo integ-test -- signal_with_start_drain

[CLAassistant]

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