fix(cli): restore previous extension on failed update

[Eswar809]

Summary

If loadExtension or any of the file operations after the destructive uninstall step (mkdir, copyExtension, metadata write, integrity check, load)
failed during an extension update, the user was left without a working extension. A pre-existing TODO at the loadExtension call site already
acknowledged this gap.

Snapshot the previous extension directory into a temp dir (via ExtensionStorage.createTmpDir()) before the destructive sequence. On any update failure,
rm -rf the partial new destination and copyExtension the snapshot back into the original location. The snapshot is unconditionally cleaned up in
finally on both success and failure paths.

The broader (uninstall → copy → metadata → integrity → load) window is covered, not just the load step the TODO pointed at — any of those steps failing
equally orphans the user.

Fixes #21671

Test plan

• Full extension test suite — 96/96 pass (no regression in existing install/update/uninstall paths)
• npm run preflight exit 0

[gemini-code-assist]

Summary of Changes

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This pull request improves the reliability of the extension update process by introducing a rollback mechanism. By creating a temporary snapshot of the current extension before applying updates, the system can now gracefully recover if any part of the installation sequence fails, preventing users from being left with a non-functional extension.

Highlights

• Robust Extension Updates: Implemented a snapshot mechanism to back up the existing extension directory before performing destructive update operations.
• Automated Rollback: Added error handling to automatically restore the previous extension version if any step in the update process fails, ensuring the user is never left with a broken state.
• Cleanup Logic: Introduced a finally block to ensure temporary backup directories are reliably removed regardless of whether the update succeeded or failed.

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