(gen2-migration) `lock` fails in Planning with "Unable to find nested stack logical id prefix" for imported auth

### How did you install the Amplify CLI?

npm (`npm install -g @aws-amplify/cli`)

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

v20 (also reproduced on v23)

### Amplify CLI Version

14.5.1

### What operating system are you using?

Mac

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

No manual changes to the Amplify-managed stack. The auth resource is an existing Cognito user pool brought in via `amplify import auth` (not created by `amplify add auth`), following the imported-auth guidance: https://docs.amplify.aws/gen1/react/tools/console/auth/import/

### Describe the bug

`amplify gen2-migration lock` fails during the **Planning** phase for a project whose auth category is an **imported** Cognito resource. This reproduces with imported auth alone — no GraphQL API or other category is required. It throws:

```
🛑 Unable to find nested stack logical id prefix: auth
→ Planning failed
```

`amplify status` reports **no changes** for the project.

This appears to be distinct from #14702. #14702 was a false-positive **Drift** failure in the **Validating** phase (fixed by #14803). The failure here happens earlier, in **Planning**, and is not skippable with `--skip-validations` (that flag only skips the Validating phase).

Observed root cause (from reading the bundled CLI 14.5.1):

- In the lock step, planning iterates over `Gen1App.discover()` and switches on `resource.key`. For auth it matches `auth:Cognito` and calls `findNestedStack(nestedStacks, "auth")`.
- An imported auth resource is not represented as a CloudFormation nested stack, so no logical id starts with that prefix, and `findNestedStack` throws.
- `discover()` derives `key` from `resourceMeta.service` only; it does not consider `resourceMeta.serviceType === "imported"`. So imported auth is treated the same as a managed Cognito resource and enters the nested-stack lookup path.

This is consistent with #14781 ("unsupported imported resources are not skipped"), which notes imported resources (excluding auth) are not identified — here the auth import specifically is not skipped in the lock planning path.

For reference, `Import auth` is listed as fully supported (🟢) in the migration Feature Parity matrix, which does not match the observed behavior: https://docs.amplify.aws/react/start/migrate-to-gen2/feature-matrix/

### Expected behavior

`amplify gen2-migration lock` should handle an imported auth resource without failing in Planning — either by skipping the nested-stack lookup for imported resources (they are not part of the Amplify-managed CloudFormation stack), or by treating imported auth the way `amplify status` does (no change), so that migration can proceed.

### Reproduction steps

1. Create a Cognito user pool + app client(s) + identity pool **outside** of Amplify (e.g. via the AWS Console or CLI).
2. In a fresh Gen 1 project (`amplify init`), bring the Cognito resources in with `amplify import auth` (imported auth, not `amplify add auth`). No other category is needed.
3. `amplify push`. Confirm `amplify status` shows no changes.
4. Install `@aws-amplify/cli@14.5.1` (migration tooling included).
5. Run `amplify gen2-migration lock`.
6. Observe the Planning-phase failure: `Unable to find nested stack logical id prefix: auth`.
7. Re-run with `amplify gen2-migration lock --skip-validations`. Observe the same Planning failure (the flag does not help because the failure is in Planning, before Validating).

### Project Identifier

_No response_

### Log output

```
→ Planning failed

🛑 Unable to find nested stack logical id prefix: auth

Learn more at: https://docs.amplify.aws/cli/project/troubleshooting/

Session Identifier: xxxxxxx
✔ An unexpected error has occurred, opt in to send an error report to AWS Amplify with non-sensitive project configuration files. Confirm (y/N)
```

### Additional information

- Reproduced on CLI 14.5.1 (the current `latest` Gen 1 CLI at the time of writing).
- Related: #14702 (Validating-phase false drift for imported auth; fixed by #14803), #14781 (imported resources not skipped).

### 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.

Start with the gen2-migration lock Planning phase, tracing Gen1App.discover() for imported auth and the findNestedStack lookup. Reproduce with an imported Cognito resource and confirm that --skip-validations does not change the Planning failure. Done means lock handles imported auth without the nested-stack error and migration can proceed.

Written by the indexing model from the issue text.