Centralize error handling in _mk_auth_req

[ncoop57]

Summary

Refactors error handling to raise PlashError with meaningful messages directly in _mk_auth_req, eliminating redundant error checks throughout the codebase.

Changes

• _mk_auth_req now raises PlashError on non-200 responses with a descriptive message (from X-Plash-Error header, response body, or HTTP status)
• Removes duplicate if not r: raise PlashError(...) checks from all API functions
• Adds tests for the new error handling behavior

Benefits

• Consistent error messages surfaced to users
• Cleaner code with single point of error handling
• Properly handles cases where X-Plash-Error header is missing

[github-actions]

📖 Documentation Preview: https://plash-docs-pr-76.pla.sh