[v2] SSOTokenLoader Expiration Check

[aemous]

Description of changes:

• Updated SSOTokenLoader to check if the cached legacy token is expired according to the local clock. If expired, it will raise an UnauthorizedSSOTokenError instead of sending an expired token to Identity Center's GetRoleCredentials API.
• Added a unit test to verify when an expired token is cached, that GetRoleCredentials is not called and UnauthorizedSSOTokenError is raised.

Description of tests:

• Ran all test suites (unit, functional, integration, etc.)
• The following manual workflow was completed to test the code.
  1. Login to a user account using the legacy flow via aws sso login.
  2. Let the cached token expire.
  3. Ran a command using the SSO profile (e.g. aws s3 ls --profile SSOProfile --debug)
  4. Verified an UnauthorizedSSOTokenError was raised, and checked the stacktrace (pasted below) to verify this exception was raised without GetRoleCredentials being called.

Stacktrace from manual workflow on this branch

 File "/Users/aemous/GitHub/aws-cli2/awscli/botocore/credentials.py", line 519, in _protected_refresh
    metadata = self._refresh_using()
               ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aemous/GitHub/aws-cli2/awscli/botocore/credentials.py", line 660, in fetch_credentials
    return self._get_cached_credentials()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aemous/GitHub/aws-cli2/awscli/botocore/credentials.py", line 670, in _get_cached_credentials
    response = self._get_credentials()
               ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aemous/GitHub/aws-cli2/awscli/botocore/credentials.py", line 2056, in _get_credentials
    raise UnauthorizedSSOTokenError()

Stacktrace from manual workflow on v2

Traceback (most recent call last):
  File "/Users/aemous/GitHub/aws-cli2/awscli/botocore/credentials.py", line 518, in _protected_refresh
    metadata = self._refresh_using()
               ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aemous/GitHub/aws-cli2/awscli/botocore/credentials.py", line 659, in fetch_credentials
    return self._get_cached_credentials()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aemous/GitHub/aws-cli2/awscli/botocore/credentials.py", line 669, in _get_cached_credentials
    response = self._get_credentials()
               ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aemous/GitHub/aws-cli2/awscli/botocore/credentials.py", line 2055, in _get_credentials
    raise UnauthorizedSSOTokenError()
botocore.exceptions.UnauthorizedSSOTokenError: The SSO session associated with this profile has expired or is otherwise invalid. To refresh this SSO session run aws sso login with the corresponding profile.
2025-03-13 10:45:14,232 - MainThread - awscli.clidriver - DEBUG - Exception caught in main()
Traceback (most recent call last):
  File "/Users/aemous/GitHub/aws-cli2/awscli/botocore/credentials.py", line 2053, in _get_credentials
    response = client.get_role_credentials(**kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aemous/GitHub/aws-cli2/awscli/botocore/client.py", line 365, in _api_call
    return self._make_api_call(operation_name, kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aemous/GitHub/aws-cli2/awscli/botocore/context.py", line 124, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aemous/GitHub/aws-cli2/awscli/botocore/client.py", line 752, in _make_api_call
    raise error_class(parsed_response, operation_name)
botocore.errorfactory.UnauthorizedException: An error occurred (UnauthorizedException) when calling the GetRoleCredentials operation: Session token not found or invalid

Notice the above exception is raised due to a returned UnauthorizedException from GetRoleCredentials, while this is not the case in the previous stacktrace.

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[ashovlin]

1. I think it's still worth a changelog entry explaining the change in behavior.
2. The Windows test failures look valid.
3. Can you add a test case asserting that the expected error was thrown and that no calls were made to GetRoleCredentials?