Go SDK: Shut down Edge Workers gracefully on SIGTERM

[viiccwen]

Handle SIGTERM through the same graceful drain path as the first interrupt received by a Go SDK Edge Worker.

The worker registered SIGTERM with signal.Notify, which replaced the signal's default termination behavior, but its signal switch did not handle SIGTERM. Container runtimes and service managers could therefore leave the worker running until escalating to a forced kill, bypassing task draining and normal lifecycle cleanup.

Separate OS signal registration from the core worker loop so tests can inject a signal channel without signaling the test process. Stop signal delivery when the wrapper exits, and route both SIGINT and SIGTERM through the existing drain behavior.

Add a regression test that injects SIGTERM and uses a test Edge API server to verify the worker reports an idle heartbeat followed by offline as it drains. The test also verifies the path repeatedly without installing process-wide signal handlers.

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Was generative AI tooling used to co-author this PR?

• Yes - GPT-5.6 Sol

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[jason810496]

Thanks for the fix.

[viiccwen]

@jason810496 politely tag for further review.

[jason810496]

Sorry, as #71874 was merged. We don't need this PR any more. Thanks for your contribution.