LCORE-1724: try AWS_STS_REGIONAL_ENDPOINTS=regional for Pulumi STS validation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: are-ces

.tekton/integration-tests/pipeline/lightspeed-stack-rhelai-test.yaml

@@ -145,6 +145,7 @@ spec:
               export AWS_ACCESS_KEY_ID="$(params.aws-access-key-id)"
               export AWS_SECRET_ACCESS_KEY="$(params.aws-secret-access-key)"
               export AWS_SESSION_TOKEN="$(params.aws-session-token)"
+              export AWS_STS_REGIONAL_ENDPOINTS=regional
               echo "[mapt] Key ID length: ${#AWS_ACCESS_KEY_ID}"
               echo "[mapt] Secret length: ${#AWS_SECRET_ACCESS_KEY}"
               echo "[mapt] Token length: ${#AWS_SESSION_TOKEN}"