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

[andyatmiami]

/ok-to-test

[thesuperzapper]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: thesuperzapper

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• workspaces/backend/OWNERS [thesuperzapper]
• workspaces/controller/OWNERS [thesuperzapper]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment