Fix audio device reopen

[fire332]

Linked issue (required)

Fixes #4805

Summary / motivation (required)

This eliminates the close & re-open audio device behavior which prevents some negative device-dependent behaviors from occurring. e.g. some bluetooth headphones cycle sleep states which causes a long delay between audio clips in a card; S/PDIF devices like AVRs have the beginning of the audio cut off as it transitions between audio formats.

Steps to reproduce (required, use N/A if not applicable)

1. Switch to bluetooth headphones that sleep when the audio device is closed (e.g. Galaxy Buds2 Pro)
2. Ensure Deck Options -> Audio -> Don't play audio automatically is OFF
3. Study a card with multiple audio clips

How to test (required)

Checklist (minimum)

• I ran ./ninja check or an equivalent relevant check locally.
• I added or updated tests when the change is non-trivial or behavior changed.

Details

None.

Before / after behavior (optional)

With the fix applied using appropriate bluetooth headphones, there's no longer a few-second long delay between audio clips and no fade-in effect on subsequent audio clips.

Risk / compatibility / migration (optional)

Going through the git blame:

• keep-open=no force mpv --keep-open=no #232: allows the idle_active property to change on playback finish. This is addressed by adding another property observer for eof_reached that calls idle_active.
• gapless-audio=no Audio pause fix #1221: fixes audio cutting off early. It's been 5 years since that PR and I can't re-produce it.

Platform-dependent delay is removed.

Due to the nature of this change, the delay between audio clips is almost completely eliminated. The implication is that cards/decks that rely on the natural delay between clips may sound jarring. Decks should not be relying on this delay in the first place, which can vary with OS, drivers, devices, etc. An option to add a delay between auto-played clips can be added for deck authors if this is an issue.

UI evidence (required for visual changes; otherwise N/A)

N/A

Scope

• This PR is focused on one change (no unrelated edits).

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Documentation build overview

📚 Anki | 🛠️ Build #32619860 | 📁 Comparing d16f95d against latest (428406d)

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± genindex.html
± autoapi/aqt/sound/index.html