feat: Enhance validation viewing functionality with event emission

- Updated the `createMarkViewedHandler` to emit an event when a validation is marked as viewed, allowing the UI to update the unviewed count dynamically.
- Modified the `useUnviewedValidations` hook to handle the new event type for decrementing the unviewed validations count.
- Introduced a new event type `issue_validation_viewed` in the issue validation event type definition for better event handling.

libs/types/src/issue-validation.ts

@@ -109,6 +109,11 @@ export type IssueValidationEvent =
       issueNumber: number;
       error: string;
       projectPath: string;
+    }
+  | {
+      type: 'issue_validation_viewed';
+      issueNumber: number;
+      projectPath: string;
     };
 
 /**