(gen2-migration): `generate` does not support `email verification link with redirect` auth trigger template

### How did you install the Amplify CLI?

_No response_

### If applicable, what version of Node.js are you using?

_No response_

### Amplify CLI Version

NA

### What operating system are you using?

NA

### Did you make any manual changes to the cloud resources managed by Amplify? Please describe the changes made.

NA

### Describe the bug

## Gen2 Migration: `CustomMessageConfirmationBucket` not handled during refactor step

### Problem

When a Gen1 app uses the **Email Verification Link with Redirect** auth capability (`confirmationRedirect`), the auth CloudFormation stack contains a `CustomMessageConfirmationBucket` S3 resource that hosts a static verification site (`index.html`, `verify.js`, `spinner.js`, `style.css`). The `verification-link.js` Lambda trigger constructs an `s3-website` URL pointing to this bucket to confirm user accounts and redirect them.

The **generate** step correctly migrates the `CustomMessage` Lambda trigger and wires it into `defineAuth({ triggers: { customMessage } })`. However, the **refactor** step has no awareness of the `CustomMessageConfirmationBucket` resource inside the auth nested stack.

This means after migration:
- The trigger Lambda is migrated and expects the bucket to exist
- The bucket itself is not moved, recreated, or referenced in the Gen2 stack
- The verification flow breaks because the bucket is orphaned or missing

### Current behavior

- `generate`: Handles the `CustomMessage` trigger function (copies source, wires env vars like `REDIRECTURL`, `EMAILSUBJECT`, `EMAILMESSAGE`, registers trigger in `defineAuth`). ✅
- `refactor`: Does not account for `CustomMessageConfirmationBucket` in the auth stack. The resource is neither moved to a holding stack nor recreated in Gen2. ❌

### Expected behavior

The refactor step should handle the `CustomMessageConfirmationBucket` S3 resource so the verification link flow continues to work after migration. This could mean:
1. Moving the bucket resource during the refactor stack migration (similar to how other auth resources are handled), or
2. Recreating the bucket and its static assets in the Gen2 backend definition, or
3. Importing the existing bucket into the Gen2 stack via a custom resource

### Relevant code

| Area | File |
|------|------|
| Bucket creation (Gen1 auth stack) | `packages/amplify-category-auth/src/provider-utils/awscloudformation/auth-stack-builder/auth-cognito-stack-builder.ts` |
| Bucket name derivation | `packages/amplify-category-auth/src/provider-utils/awscloudformation/utils/verification-bucket-name.ts` |
| Asset upload to bucket | `packages/amplify-category-auth/src/provider-utils/awscloudformation/utils/trigger-file-uploader.ts` |
| Static site assets | `packages/amplify-category-auth/provider-utils/awscloudformation/triggers/CustomMessage/assets/` |
| Lambda trigger (consumes bucket URL) | `packages/amplify-category-auth/provider-utils/awscloudformation/triggers/CustomMessage/verification-link.js` |
| Gen2 migration generate (trigger handling) | `packages/amplify-cli/src/commands/gen2-migration/generate/amplify/function/function.generator.ts` |
| Gen2 migration refactor (no bucket handling) | `packages/amplify-cli/src/commands/gen2-migration/refactor/` |

### Additional context
- The bucket name follows the pattern `{authResourceName}verificationbucket-{env}` and is stored in `cli-inputs.json` as `verificationBucketName`.

### Expected behavior

NA

### Reproduction steps

NA

### Project Identifier

_No response_

### Log output

```
# Put your logs below this line

```

### Additional information

_No response_

### Before submitting, please confirm:

- [x] I have done my best to include a minimal, self-contained set of instructions for consistently reproducing the issue.
- [x] I have removed any sensitive information from my code snippets and submission.