Implement termination grace period support for the runner

[TingluoHuang]

export ACTIONS_RUNNER_TERMINATION_GRACE_PERIOD_SECONDS=100

The runner will not cancel its current runner step during the grace period.

However, depends on how child process launched in the step, those process might still exit by itself if they dont' trap '' INT TERM.

Try to improve: #3308

[Copilot]

Pull Request Overview

This PR adds support for a configurable termination grace period for the runner, allowing it to finish its current job before shutting down.

• Detect and flag a non‑empty ACTIONS_RUNNER_TERMINATION_GRACE_PERIOD_SECONDS environment variable
• Introduce GetShutdownDelay() to compute and apply a shutdown delay when the runner is busy
• Extend HostContext.ShutdownRunner to accept an optional delay and wire it into unload/Ctrl+C logic

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Runner.Listener/Runner.cs	Read and store the grace‑period env var; wrap shutdown calls with GetShutdownDelay(); add job‑status handler
src/Runner.Common/HostContext.cs	Change ShutdownRunner signature to accept a delay; apply CancelAfter(delay) when set
src/Runner.Common/Constants.cs	Add ACTIONS_RUNNER_TERMINATION_GRACE_PERIOD_SECONDS constant

Comments suppressed due to low confidence (2)

src/Runner.Listener/Runner.cs:843

• Consider adding unit tests for GetShutdownDelay() to cover scenarios where the runner is busy versus not busy, valid and invalid env var values, and boundary conditions for the delay range.

private TimeSpan GetShutdownDelay()

src/Runner.Listener/Runner.cs:317

• [nitpick] Enhance this log message by including the actual grace period value (e.g. seconds) to make debugging easier when the variable is set.

Trace.Verbose($"Runner has termination grace period set");

[TingluoHuang]

Change to GetService, so i can fetch the same instance in the event handler.
We never need more than 1 jobDispatcher in the listener, so switching to Get should be safe.

[TingluoHuang]

i use _hasTerminationGracePeriod as the feature flag, so unless the new ENV is set, no new code will get executed.