fix(image_uri): add eusc-de-east-1 to ALTERNATE_DOMAINS

test_djl_uris compares image_uris.retrieve() against a helper that
derives the ECR domain from ALTERNATE_DOMAINS. eusc-de-east-1 was
missing, so the helper fell back to amazonaws.com while retrieve()
emitted amazonaws.eu (botocore aws-eusc partition), failing the test.
Add eusc-de-east-1 to amazonaws.eu, mirroring the eu-isoe-west-1 entry.
Unblocks the eusc entries added to djl-lmi and huggingface-llm-neuronx.

Author: Eli Davidson

sagemaker-core/src/sagemaker/core/common_utils.py

@@ -62,6 +62,7 @@
     "us-isof-south-1": "csp.hci.ic.gov",
     "us-isof-east-1": "csp.hci.ic.gov",
     "eu-isoe-west-1": "cloud.adc-e.uk",
+    "eusc-de-east-1": "amazonaws.eu",
 }
 
 ECR_URI_PATTERN = r"^(\d+)(\.)dkr(\.)ecr(\.)(.+)(\.)(.*)(/)(.*:.*)$"