bug: Gracefully handle failed extension updates in extension-manager.ts

### What happened?

In `packages/cli/src/config/extension-manager.ts`, when upgrading/updating an extension, the files are copied directly to the `destinationPath` (overwriting the existing extension files). Right after copying, the manager attempts to load the new extension:
```typescript
// TODO: Gracefully handle this call failing, we should back up the old
// extension prior to overwriting it and then restore and restart it.
extension = await this.loadExtension(destinationPath);
```
If `loadExtension` throws an error (e.g. because of a syntax error in the updated JavaScript/TypeScript files, a malformed config, or missing files), the update process fails, but the old working version of the extension has already been deleted and overwritten. The user is left with a broken/unusable extension.

### What did you expect to happen?

Before copying the update over an existing extension, the extension manager should create a backup of the old extension directory. If `loadExtension` fails to load the update, the manager should catch the error, restore the backup, and reload the previous working version while logging/displaying a graceful error message to the user.

### Client information

Platform: All
Core version: 0.51.0

### Anything else we need to know?

Relevant code segment:
https://github.com/google-gemini/gemini-cli/blob/main/packages/cli/src/config/extension-manager.ts#L450

Start in packages/cli/src/config/extension-manager.ts around the update flow and the loadExtension(destinationPath) call. Trace how the existing extension is overwritten and how loading failures are reported. Done means a failed update preserves and reloads the previous working extension while providing a graceful error to the user.

Written by the indexing model from the issue text.