Harden data safety: awaited saves, atomic writes, corruption recovery, rolling backups

[tahabasri]

What

The extension's most serious recurring bug class is silent data loss (#31, #33, #66, #101, #119, #129). This PR hardens the whole persistence path:

• Fix handleCommand: new Promise(callback) treated command bodies as executors whose resolve/reject were never called — the promise never settled and async command errors bypassed the handler entirely. Now chains through Promise.resolve().then(callback) so rejections are surfaced to the user and logged with a stack.
• Awaited saves end-to-end: memento.update() was fire-and-forget, so save failures were silently swallowed and the tree refreshed from a state that never committed. DataAccess.save is now async and SnippetsProvider.sync() awaits it; all provider mutations propagate the save promise.
• Atomic file writes: FileDataAccess writes to a temp file then renames, so a crash mid-write can never truncate .vscode/snippets.json.
• Corruption recovery: an unparseable snippets file no longer kills activation — the persistent workspace backend preserves it as *.corrupt-<timestamp>, reseeds an empty root and notifies the user. Transient import instances keep throwing so a bad import source is never overwritten.
• Rolling automatic backups (new BackupManager): timestamped snapshots (10 kept per view) in global storage before destructive operations (import, troubleshoot, folder deletion) plus at most one automatic snapshot per day. New Snippets: Open Backups Folder command makes them discoverable.

Tests

• New suites for atomic saves, corruption recovery (both modes) and backup rotation/pruning; existing suites updated for the async signatures. 49 tests passing locally.
• @vscode/test-electron bumped 2.3.4 → 2.5.2 (the old version cannot launch current VS Code stable builds).

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