feat: add structured error handling with AuthError type

[grdsdev]

Summary

Implements Phase 1.1 of the roadmap by adding structured error handling to the auth-go library.

Changes

• ✅ Add AuthError type in types/error.go with StatusCode, Message, ErrorCode, and Details fields
• ✅ Add handleErrorResponse helper function in endpoints/errors.go to parse API error responses
• ✅ Update all 23 endpoint files to use the new error handling instead of fmt.Errorf
• ✅ Remove unused imports (fmt, io) from endpoint files
• ✅ Preserve backward-compatible error message format ("response status code %d: %s")

Backward Compatibility

This is a non-breaking change:

• ✅ Errors still implement the error interface
• ✅ Error message format matches the previous format exactly
• ✅ Existing code continues to work without modification
• ✅ Users can now opt-in to structured error inspection using errors.As()

Usage Example

resp, err := client.SignUp(req)
if err != nil {
// Still works (error is still an error, same message format)
log.Printf("Error: %v", err)

// NEW: Can now inspect structured errors
var authErr *types.AuthError
if errors.As(err, &authErr) {
    if authErr.StatusCode == 429 {
        // Handle rate limiting
    }
    log.Printf("Error code: %s", authErr.ErrorCode)
    log.Printf("Details: %v", authErr.Details)
}

}