CI-21342: migrate AWS SDK Go v1 to v2 for ECR service

[chhawchharia]

Summary

• Migrate AWS SDK Go v1 (github.com/aws/aws-sdk-go) to v2 (github.com/aws/aws-sdk-go-v2) for ECR authentication
• Replace session.NewSession() with config.LoadDefaultConfig(ctx) and ecr.NewFromConfig(cfg)
• Replace temp-file-based OIDC token handling with in-memory identityToken type
• Add defensive bounds checks in getAuthInfo for AuthorizationData slice
• Update error handling to use errors.As with typed exceptions

Test plan

• go build ./... passes
• go vet ./... passes
• go test ./... passes
• Verify ECR authentication works in CI environment
• Verify OIDC token flow works without temp file

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