refactor: use Zod validation for executionError getter

- Export zExecutionErrorWsMessage schema from apiSchema
- Use safeParse instead of type casts in TaskItemImpl.executionError
- Add logging on validation failure for debugging
- Keep errorMessage extraction in useJobErrorReporting via the getter

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/stores/queueStore.ts

@@ -322,14 +322,6 @@ export class TaskItemImpl {
     return this.status?.messages || []
   }
 
-  /**
-   * Extracts the execution error message from status messages.
-   * Used by error reporting UI components.
-   */
-  get errorMessage(): string | undefined {
-    return this.executionError?.exception_message
-  }
-
   /**
    * Extracts the full execution error from status messages.
    * Returns the ExecutionErrorWsMessage for detailed error dialogs.