fix: update DefaultAwsCredentialsManager tests for new AWS SDK internals

The AWS SDK now uses @aws-sdk/nested-clients/sts internally in
fromTemporaryCredentials, so the tests need to mock both the regular
and nested STS clients. Also adapts to the new $source metadata on
credentials and the new @smithy/shared-ini-file-loader for ini profile
mocking.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Fredrik Adelöw <freben@spotify.com>

yarn.lock

@@ -4429,6 +4429,7 @@ __metadata:
     "@backstage/config-loader": "workspace:^"
     "@backstage/errors": "workspace:^"
     "@backstage/test-utils": "workspace:^"
+    "@smithy/shared-ini-file-loader": "npm:^4.4.6"
     aws-sdk-client-mock: "npm:^4.0.0"
     aws-sdk-client-mock-jest: "npm:^4.0.0"
   languageName: unknown