Change default environment to cloud-agnostic

[hectorcast-db]

Summary

This PR updates the SDK to be cloud-agnostic by removing cloud-specific detection and configuration. This includes updating the default environment handling and unifying Private Link error messages to work across all cloud providers without requiring cloud detection.

Changes

Environment Handling

• Add Cloud.UNKNOWN enum value for cloud-agnostic hosts
• Update DEFAULT_ENVIRONMENT to use Cloud.UNKNOWN with no dns_zone
• Make dns_zone optional in DatabricksEnvironment dataclass
• Update environment detection logic to handle None dns_zone values
• Move DEFAULT_ENVIRONMENT to end of ALL_ENVS to ensure it's used as a fallback only

Private Link Error Handling

• Unified Private Link error messages across all clouds (AWS, Azure, GCP)
• Removed cloud detection dependency from _get_private_link_validation_error()
• Single error message now provides guidance for all three cloud providers
• Removed cloud-specific _PrivateLinkInfo classes and mapping

Motivation

This change enables better support for:

• Cloud-agnostic Databricks endpoints (e.g., unified hosts, aliased hosts)
• Unknown or custom host configurations that don't match standard cloud patterns
• Future host types that don't fit AWS/Azure/GCP categorization
• Simplified error handling that doesn't require knowing the user's cloud provider upfront

Backward Compatibility

✅ This change is backward compatible:

• All existing AWS/Azure/GCP hosts continue to be detected correctly
• Only unknown/unrecognized hosts are affected
• The is_azure, is_gcp, and is_aws properties work correctly with Cloud.UNKNOWN (all return False)
• azure_workspace_resource_id still correctly sets is_azure=True regardless of detected cloud
• Private Link error messages now include information for all clouds, making them more helpful regardless of the user's environment

Testing

• Added test_unknown_cloud_environment_properties - verifies UNKNOWN cloud behavior
• Added test_azure_resource_id_sets_is_azure_with_unknown_environment - verifies Azure resource ID override works
• Updated test_get_api_error[private_link_validation_error] - verifies unified Private Link error message
• All existing tests pass

[github-actions]

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Inputs:

• PR number: 1269
• Commit SHA: a2c1959185523333e06aaf992a4f159593fb8358

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