feat: Implement auto-capture of screenshots and diagnostics on test failures

- Added a new helper function `CaptureHarnessScreenshot` to encapsulate SDL screenshot logic.
- Updated `ImGuiTestHarnessServiceImpl::Screenshot` to utilize the new screenshot helper.
- Enhanced `TestManager::CaptureFailureContext` to automatically capture screenshots and widget state on test failures.
- Introduced new fields in the `GetTestResultsResponse` proto for screenshot path, size, failure context, and widget state.
- Updated CLI and gRPC client to expose new diagnostic fields in test results.
- Ensured that screenshots are saved in a structured directory under the system's temp directory.
- Improved logging for auto-capture events, including success and failure messages.

src/cli/service/gui_automation_client.h

@@ -119,6 +119,10 @@ struct TestResultDetails {
   std::vector<AssertionOutcome> assertions;
   std::vector<std::string> logs;
   std::map<std::string, int> metrics;
+  std::string screenshot_path;
+  int64_t screenshot_size_bytes = 0;
+  std::string failure_context;
+  std::string widget_state;
 };
 
 enum class WidgetTypeFilter {