refactor(copilot): improve code quality in authentication module

- Add Unwrap() to AuthenticationError for proper error chain handling with errors.Is/As
- Extract hardcoded header values to constants for maintainability
- Replace verbose status code checks with isHTTPSuccess() helper
- Remove unused ExtractBearerToken() and BuildModelsURL() functions
- Make buildChatCompletionURL() private (only used internally)
- Remove unused 'strings' import

internal/auth/copilot/errors.go

@@ -55,6 +55,11 @@ func (e *AuthenticationError) Error() string {
 	return fmt.Sprintf("%s: %s", e.Type, e.Message)
 }
 
+// Unwrap returns the underlying cause of the error.
+func (e *AuthenticationError) Unwrap() error {
+	return e.Cause
+}
+
 // Common authentication error types for GitHub Copilot device flow.
 var (
 	// ErrDeviceCodeFailed represents an error when requesting the device code fails.