fix(backend): Refactor S3 bucket management by replacing MinIO client code with AWS SDK v2 and improving configuration validation

[hbelmiro]

Problem

The API server required OBJECTSTORECONFIG_ACCESSKEY and OBJECTSTORECONFIG_SECRETACCESSKEY to be non-empty, which broke AWS IRSA (IAM Roles for Service Accounts) authentication where credentials are obtained from the environment.

Solution

• Make AccessKey and SecretAccessKey optional in validation
• When credentials are empty, use the AWS SDK's default credential chain (supports IRSA, environment variables, instance profiles)
• Replace minio-go with AWS SDK v2 for bucket operations (consistent SDK usage)
• Only suppress BucketAlreadyOwnedByYou errors; BucketAlreadyExists now surfaces as a warning to help detect misconfiguration

Changes

• validateRequiredConfig: No longer requires credentials; only validates both are set or both empty
• loadAWSConfig: Uses default credential chain when credentials are empty
• ensureBucketExists: Refactored to use AWS SDK v2 instead of minio-go
• isBucketAlreadyOwnedByUs: Only suppresses BucketAlreadyOwnedByYou (concurrent creation race), surfaces BucketAlreadyExists as warning

Fixes:

• [backend] KFP 2.15.0 breaks IRSA on AWS: AccessKey is now required #12508

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