[Awake] Fix for countdown timer drift

[daverayment]

Summary of the Pull Request

• Fixes the countdown timer drift issue [Awake] Timed keep-awake may show incorrect time remaining in the system tray #41671
• Includes minor refactoring to consolidate identical timer completion code in SetExpirableKeepAwake and SetTimedKeepAwake.
• Removes the ~50 day restriction on timed keep-awake. The timer may now be uint.MaxValue seconds, or ~136 years.

PR Checklist

• Closes: [Awake] Timed keep-awake may show incorrect time remaining in the system tray #41671
• Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end-user-facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places
  • JSON for signing for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

This replaces the combined Observable.Timer and Observable.Interval timers with a single 1-second Interval timer which checks against a fixed expiry time.

Validation Steps Performed

Checked that:

1. The timed keep-awake works via the --time-limit parameter, and expiry occurs on time.
2. The countdown timer in the systray menu correctly counts down for small values:

3. The countdown timer in the systray menu counts down for larger values than were previously possible:

4. On a heavily CPU-loaded system, the previous countdown drift does not happen.
5. The expirable keep-awake mode still functions as expected.

[Copilot]

Pull Request Overview

This PR fixes countdown timer drift issues in the Awake module by replacing the combined Observable.Timer and Observable.Interval approach with a single interval timer that checks against a fixed expiry time.

• Consolidates duplicate timer completion logic into a shared HandleTimerCompletion method
• Removes the ~50 day timer limit, allowing up to ~136 years (uint.MaxValue seconds)
• Fixes timer drift by using absolute time comparisons instead of cumulative intervals

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[Copilot]

The condition remaining.TotalSeconds > 0 will exclude the case where exactly 0 seconds remain, potentially causing the timer to miss the final update before completion. Consider using >= 0 to ensure the timer displays '0 seconds remaining' before completing.

Suggested change

	.TakeWhile(remaining => remaining.TotalSeconds > 0)
	.TakeWhile(remaining => remaining.TotalSeconds >= 0)

[daverayment]

At zero seconds, the full duration will have expired and the application will immediately end the timed mode.

The next thing that would happen is that the application would exit completely (if in "standalone" mode), or would enter the Passive mode, which updates the system tray icon straight away, removing the hover text.

The "00d 00h 00m 00s" countdown timer would likely never be seen.

[Copilot]

Casting TotalSeconds to uint will truncate negative values to unexpected large numbers if the timer has already expired. Since TakeWhile should prevent negative values, consider adding Math.Max(0, remainingTimeSpan.TotalSeconds) for safety.

Suggested change

	TimeRemaining = (uint)remainingTimeSpan.TotalSeconds;
	TimeRemaining = (uint)Math.Max(0, remainingTimeSpan.TotalSeconds);

[daverayment]

The TakeWhile clause for the Observable.Interval is remaining.TotalSeconds > 0. It is therefore not possible for the "tick" code to be executed with a negative value for TimeRemaining.

        Observable.Interval(TimeSpan.FromSeconds(1))
            .Select(_ => targetExpiryTime - DateTimeOffset.Now)
            .TakeWhile(remaining => remaining.TotalSeconds > 0)    // condition - there must be time remaining
            .Subscribe(
                remainingTimeSpan =>
                {
                    // At this point, the seconds remaining cannot be negative.
                    TimeRemaining = (uint)remainingTimeSpan.TotalSeconds;