[gen2-migration] Adopted Cognito User Pool: generator includes immutable UsernameConfiguration in the update changeset

## Summary
When adopting a pre-existing Cognito User Pool, the generated auth stack includes `UsernameConfiguration`, which is create-only in CloudFormation, so the update deploy fails.

## Error
`[CFNUpdateNotSupportedError] ... Invalid request provided: Updates are not allowed for property - UsernameConfiguration.`

## Details
The codegen emits `UsernameConfiguration` as a raw passthrough for adopted pools; there is no automatic `addPropertyDeletionOverride('UsernameConfiguration')` for the adopted-pool case. (In some MFA/SMS configurations the adopt-time path also emits duplicate `UsernameConfiguration` deletion overrides — same root cause: the generator does not reconcile immutable properties for an adopted pool.)

## Expected
For an adopted pool, immutable properties such as `UsernameConfiguration` should be omitted from the update changeset automatically.

## Type
Bug / blocker

Start at the gen2-migration auth code generator and trace the adopted-pool path that emits UsernameConfiguration. Check how immutable-property deletion overrides are handled, especially for MFA/SMS configurations. Done means adopted-pool update changesets omit UsernameConfiguration and do not contain duplicate deletion overrides.

Written by the indexing model from the issue text.