Suppress delete confirmation when sync removes tasks from deselected lists

[desimpson]

Deselecting a Google Tasks list in settings would trigger the delete confirmation modal for every removed task on the next sync. These removals are sync-initiated, not user-initiated, and should not prompt.
This fix wraps sync jobs with an isSyncInProgress flag and a post-sync cache refresh so the file modification handler skips deletion detection for sync-driven changes.

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Note

Medium Risk
Touches deletion-detection logic that can lead to missed or extra delete prompts, which affects whether Google Tasks deletions are user-confirmed. Scope is contained and backed by new integration tests, but timing/guarding around vault modify events is inherently concurrency-sensitive.

Overview
Prevents delete-confirmation modals from appearing when the sync itself removes tasks from the Markdown note (e.g., after a Google Tasks list is deselected).

Sync jobs are now wrapped in a new SyncGuard so handleFileModification can snapshot syncGuard.isActive and skip deletion detection during sync-driven note edits, while still updating the file-content cache.

After each sync job completes, the plugin refreshes the file-content cache before releasing the guard to avoid false deletion detection from delayed modify events; integration tests were added to cover both in-sync and post-sync event timing.

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