test: harden process and TUI synchronization

[dejay-vu]

Summary

• reject non-positive process IDs before probing for process exit
• treat only ProcessLookupError as evidence that the process is gone
• fail explicitly when os.kill(pid, 0) raises another OSError
• wait for the asynchronous GC preview callback before inspecting its modal

Why

The process-exit helper follows up on Copilot feedback from PR #3: catching every
OSError could incorrectly pass when a process still existed but could not be
inspected. The Python 3.14 check also exposed a separate TUI race where the test
could inspect the screen before the GC callback had pushed the confirmation
modal.

Impact

Production behavior is unchanged. These tests now distinguish a missing process
from a failed process probe and synchronize with both the GC callback and modal
mounting before making assertions.

Validation

• GC preview test passed 20 consecutive runs
• targeted background login-build test passed
• conda run -n slurmdeck-dev ruff format .
• conda run -n slurmdeck-dev ruff check .
• conda run -n slurmdeck-dev mypy
• conda run -n slurmdeck-dev python -m pytest -q (510 passed)
• git diff --check

[Copilot]

Pull request overview

This PR hardens timing- and OS-error-sensitive test helpers by making process-exit probing distinguish “process is gone” from “probe failed”, and by synchronizing a TUI test with the GC preview confirmation modal before asserting on it.

Changes:

• Reject non-positive PIDs and treat only ProcessLookupError as a definitive “process exited” signal during os.kill(pid, 0) probing.
• Fail explicitly when os.kill(pid, 0) raises other OSError subclasses (e.g., permission/probing errors) instead of silently passing.
• Add an explicit async wait for the ConfirmModal screen before inspecting/clicking it in the GC preview TUI test.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/unit/test_env_executors.py	Tightens process-exit polling to avoid false positives and unsafe PID values in unit tests.
tests/tui/test_pilot.py	Adds explicit modal synchronization to prevent a race when asserting against the GC confirmation UI.

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