chore: improved validation and error responses

[andyatmiami]

ℹ️ : NO GH ISSUE

chore: improve validation and error response handling

This commit delivers improvements to validation and error handling across the backend API.

Key changes:

Error Response Improvements:

• Enhanced error response structure with origin tracking (INTERNAL vs KUBERNETES)
• Added support for extracting StatusCauses from both Invalid and Conflict Kubernetes errors via StatusCausesFromAPIStatus helper
• Improved conflict error handling to always include origin field in API responses, ensuring pause action conflicts show KUBERNETES origin
• Added comprehensive test coverage for error response functions

Validation Enhancements:

• Refactored workspace model types (types_create.go -> types_write.go) for better code organization
• Enhanced validation helper functions with improved error extraction
• Added IsEOFError handling for better empty request body detection
• Improved webhook validation for WorkspaceKind resources

Handler Updates:

• Updated PauseActionWorkspaceHandler to properly handle Invalid errors from Kubernetes patch operations, ensuring conflict_causes[] includes KUBERNETES origin
• Enhanced CreateWorkspaceHandler and other handlers to utilize improved error extraction and response formatting
• Updated workspace and workspacekind handlers to use new validation and error response patterns

Testing:

• Added comprehensive test suite for response_errors.go
• Added extensive validation helper tests
• Updated existing handler tests to match new error response format

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[andyatmiami]

/ok-to-test