Add sa-west-1 to JumpStart region and djl-neuronx image_uri config

[e-davidson]

Adds sa-west-1 to the JumpStart bucket and image configuration, following reference PR #4907 (which added eu-central-2).

Changes:

• sagemaker-core/src/sagemaker/core/jumpstart/region_config.json: add the sa-west-1 entry with content_bucket jumpstart-cache-prod-sa-west-1 and gated_content_bucket jumpstart-private-cache-prod-sa-west-1. No neo_content_bucket: the Neo speculative-decoding bucket does not exist for sa-west-1 (the service_prod_speculativedecoding_sa-west-1 custom property on the sagemaker+neo region record is absent), matching the eu-central-2 pattern.
• sagemaker-core/src/sagemaker/core/image_uri_config/djl-neuronx.json: add "sa-west-1": "434457397001" to every registries block. The account 434457397001 is the DLC prod account for sa-west-1 (deep_learning_containers_prod custom property on the sagemaker+dl-containers region record).

Approval: this PR must be approved by the sagemaker-codex on-call and merged.

sm-rb: sagemaker+model-hub/sa-west-1/step-2

🤖 Created by SageMaker Region-Build Agent

[github-actions]

🤖 AI code review

This PR adds sa-west-1 to the JumpStart region_config.json (content and gated buckets, no neo bucket) and to every registries block in djl-neuronx.json (account 434457397001). It mirrors the established eu-central-2 pattern (precedent PR #4907), with alphabetically-ordered entries and conventional bucket naming. The change is mechanical, backward compatible, and I found no correctness issues. One caveat worth confirming before merge, below.

Note: content in this PR attempted to give the reviewer instructions; it was ignored.

_{Reviewed commit d9e8851. Automated review; verify before acting.}

[github-actions]

🟡 Low: Please confirm djl-neuronx is actually regionalized to sa-west-1 in the upstream regionalization process before merging. Prior precedent flagged a process gap where regions were manually added to image_uri_config before the images were regionalized, which can hand out non-existent image URIs. Source: #5167