fix(smus): Make AWS partition and endpoint handling GovCloud-resilient

[ziwikiwi]

Local IDE hardcoded the commercial aws partition and .amazonaws.com DNS suffix in several ARN and endpoint constructions, which break in GovCloud (aws-us-gov partition). Route these through the existing regionProvider (getPartitionId / getDnsSuffixForRegion) so they resolve correctly per partition.

• smusUtils.extractAccountIdFromSageMakerArn: match any partition (arn:aws[a-z-]*:) instead of literal arn:aws:
• detached-server parseArn: match any partition
• detached-server startSagemakerSession: only override the endpoint when SAGEMAKER_ENDPOINT is set; otherwise let the SDK resolver derive the partition-correct host (this file cannot import vscode/globals)
• sqlWorkbenchClient: derive partition for the ARN and dnsSuffix for the endpoint from regionProvider
• auth/sso OidcClient.authorize: derive dnsSuffix for the OIDC endpoint

Added GovCloud (aws-us-gov) test cases for both ARN parsers.

Note: two hardcoded endpoints were intentionally left for follow-up as they need endpoint-format confirmation, not a mechanical fix:

• datazoneCustomClientHelper datazone.{region}.api.aws
• smusUtils Identity Center issuer identitycenter.amazonaws.com (coupled to a validation regex in auth/sso/constants.ts)

Problem

Solution

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[amazon-inspector-ohio]

⏳ I'm reviewing this pull request for security vulnerabilities and code quality issues. I'll provide an update when I'm done

[amazon-inspector-ohio]

✅ I finished the code review, and didn't find any security or code quality issues.

[github-actions]

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