…uffix (#3024)

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## Problem
When using imported Cognito resources in Amplify Gen2 with SSO enabled,
login redirects were broken because the initializer Lambda
unconditionally appended `.auth.{region}.amazoncognito.com` to the OAuth
domain.
This caused malformed redirect URLs when a custom domain was already set
in the Cognito User Pool (e.g., `auth.dev.example.com` →
`auth.dev.example.com.auth.us-east-1.amazoncognito.com`).
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**Issue number, if available:** #2991 

## Changes

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- Updated getUserPoolOutputs logic so that:
  - If a custom domain is provided, it is used as-is.
- Otherwise, fallback to Cognito-managed domain
({domain}.auth.{region}.amazoncognito.com).
- Ensures fullDomainPath is properly constructed in both scenarios.
This fixes the malformed OAuth redirect URLs when signing in via SSO
providers (e.g., Google).

## Validation

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- Manually tested with imported Cognito resources and a custom domain
(auth.dev.example.com) → redirect now works correctly.
- Verified fallback behavior with Cognito-managed domains continues to
work.
- Confirmed <Authenticator> still detects Google as an IdP and completes
the sign-in flow.

## Checklist

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- [ ] If this PR includes a functional change to the runtime behavior of
the code, I have added or updated automated test coverage for this
change.
- [ ] If this PR requires a change to the [Project Architecture
README](../PROJECT_ARCHITECTURE.md), I have included that update in this
PR.
- [ ] If this PR requires a docs update, I have linked to that docs PR
above.
- [ ] If this PR modifies E2E tests, makes changes to resource
provisioning, or makes SDK calls, I have run the PR checks with the
`run-e2e` label set.

_By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license._

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Co-authored-by: Lucas Leonardo Soto <[email protected]>
Co-authored-by: Lucas Leonardo Soto <[email protected]>