fix(python): avoid interpreter shutdown panic in BackgroundExecutor

[Xuanwo]

Summary

• harden BackgroundExecutor against Python interpreter finalization by replacing unconditional Python::attach(...) calls with Python::try_attach(...) in spawn, spawn_background, block_on, and signal polling loops
• keep behavior the same when Python is attachable, and safely degrade to Ok(()) signal checks when Python is finalizing/unavailable
• add a regression test that runs take_blobs + close in a subprocess and asserts no interpreter_lifecycle panic appears on process shutdown

Why

take_blobs could trigger a panic during interpreter shutdown because signal checks in the executor always called Python::attach(...), which panics when Python is finalizing.
This fix addresses the root cause globally in the executor instead of adding blob-specific special handling.

Testing

• cargo check --manifest-path python/Cargo.toml --tests
• cd python && uv sync --extra tests
• cd python && uv run python -m pytest -q python/tests/test_blob.py

[github-actions]

PR Review: fix(python): avoid interpreter shutdown panic in BackgroundExecutor

Overall: LGTM ✅

This is a clean fix for the interpreter shutdown panic. Replacing Python::attach() with Python::try_attach() is the correct approach to gracefully handle the Python finalization state.

Reviewed aspects:

• Code quality: The pattern used (let mut task = Some(task); ... task.take()) is the idiomatic workaround for the borrow checker with FnOnce closures. Changes are consistent across all four call sites.
• Correctness: Falling back to Ok(()) for signal checks during shutdown is safe—we can't observe KeyboardInterrupt when Python is finalizing anyway.
• Test coverage: The subprocess-based test is the right approach for testing interpreter shutdown behavior.

No P0/P1 issues found.