chore(auth): Refresh Token Rotation Test Fix

[ekjotmultani]

Issue #

Description

The integration test testSuccessfulForceSessionFetch was failing with the current implementation of the feature due to setting refresh tokens in Amplify to nil if returned that from the API, even though this is an invalid state for us. This PR fixes that by re using the old token in this case which is what is expected in the USER_SRP flow.

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Documentation update for the change if required
• PR title conforms to conventional commit style
• New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
• If breaking change, documentation/changelog update with migration instructions

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.20%. Comparing base (fa5907b) to head (74a9d88).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4055       +/-   ##
===========================================
+ Coverage   67.77%   85.20%   +17.43%     
===========================================
  Files        1129       14     -1115     
  Lines       52473      453    -52020     
===========================================
- Hits        35562      386    -35176     
+ Misses      16911       67    -16844     

Flag	Coverage Δ
API_plugin_unit_test	?
AWSPluginsCore	?
Amplify	?
Analytics_plugin_unit_test	85.20% <ø> (ø)
Auth_plugin_unit_test	?
DataStore_plugin_unit_test	?
Geo_plugin_unit_test	?
Logging_plugin_unit_test	?
Predictions_plugin_unit_test	?
PushNotifications_plugin_unit_test	?
Storage_plugin_unit_test	?
unit_tests	85.20% <ø> (+17.43%)	⬆️

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