Stabilize iOS alert accept flow when alert is present but not yet actionable

[thymikee]

Summary

alert accept can return alert not found while an alert is still present in immediate snapshot -i output.

Version

• agent-device 0.7.6

Repro pattern

1. Open app that triggers iOS permission alert.
2. Run alert accept --platform ios --session <id> --json.
3. Sometimes get:
   • code: COMMAND_FAILED
   • message: alert not found
4. Immediately run snapshot -i and alert nodes are still present.

Expected

More robust alert handling, e.g.:

• short internal retry window before failing
• clear distinction in error details between "not visible yet" vs "no alert in hierarchy"

Why this matters

Permission alerts are frequent in clean simulator sessions and currently introduce flaky setup flows.