fix: Handle signal registration outside the main thread

[brightsparc]

Summary

• tolerate ValueError when an event loop rejects add_signal_handler()
• add an ASGI health-request regression test that reproduces the rejection without depending on uvloop internals

Root cause

Restate installs a SIGTERM handler while serving the ASGI application. The existing code handles NotImplementedError and RuntimeError, which covers unsupported platforms and asyncio's behavior outside the main thread.

uvloop raises ValueError instead when add_signal_handler() is called outside the main thread. Granian runs each ASGI worker on its own thread, so Restate requests failed with:

ValueError: add_signal_handler() can only be called from the main thread

This was observed with Granian, uvloop, and free-threaded CPython 3.14.6. Switching Granian to the asyncio loop avoided the exception, but lost the desired uvloop runtime.

Impact

The additional exception handling lets Restate applications run under Granian worker threads with uvloop, including free-threaded Python deployments. Signal registration remains best-effort, matching the existing behavior for asyncio and unsupported platforms.

No Granian-specific or uvloop-specific runtime patch is introduced.

Validation

• branch rebased on current restatedev/sdk-python:main
• git diff --check passed
• regression test added for a successful GET /restate/health when signal registration raises ValueError
• local pytest execution was attempted, but the editable Rust extension build stalled in cargo metadata at zero CPU due to unrelated long-running Cargo jobs on the development machine; upstream CI should execute the focused test

[igalshilman]

Looks good!, thank you @brightsparc !