Refactor mocking of credentials resolvers for user agent tests

[ubaskota]

This PR reverts the previously reverted PR and updates the functional tests to provide only one credential provider in a credential resolver.

Description:
In the reverted PR, we initially mocked the functional tests using get_credentials to return a list containing only one credential. This resulted in a list where the specified credential was included, but None values were returned for other credential providers. While this worked as expected in local environments, it could cause issues when lower-precedence credential providers are available.

To address this, the PR now mocks the create_credential_resolver function instead, returning a list that contains only the intended credential provider.

Tests:
I verified that the functional tests related to registering credential feature ids run successfully.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.17%. Comparing base (8121342) to head (63fc87e).
⚠️ Report is 9 commits behind head on develop.
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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3580   +/-   ##
========================================
  Coverage    93.17%   93.17%           
========================================
  Files           68       68           
  Lines        15411    15411           
========================================
  Hits         14359    14359           
  Misses        1052     1052           

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