fix: clear prompt after task creation (#20344)

Fix https://github.com/coder/coder/issues/20255

site/src/modules/tasks/TaskPrompt/TaskPrompt.stories.tsx

@@ -145,6 +145,11 @@ export const OnSuccess: Story = {
 			const successMessage = await body.findByText(/task created/i);
 			expect(successMessage).toBeInTheDocument();
 		});
+
+		await step("Clears prompt", async () => {
+			const prompt = await canvas.findByLabelText(/prompt/i);
+			expect(prompt).toHaveValue("");
+		});
 	},
 };
 

site/src/modules/tasks/TaskPrompt/TaskPrompt.tsx

@@ -232,6 +232,7 @@ const CreateTaskForm: FC<CreateTaskFormProps> = ({ templates, onSuccess }) => {
 			await createTaskMutation.mutateAsync({
 				prompt,
 			});
+			setPrompt("");
 		} catch (error) {
 			const message = getErrorMessage(error, "Error creating task");
 			const detail = getErrorDetail(error) ?? "Please try again";