test: App mode - setting widget value test (#10746)

## Summary

Adds a test for setting various types of widgets in app mode, then
validating the /prompt API is called with the expected values

## Changes

- **What**: 
- extract duplicated enableLinearMode
- add AppModeWidgetHelper for setting values

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10746-test-App-mode-setting-widget-value-test-3336d73d365081739598fb5280d0127e)
by [Unito](https://www.unito.io)

browser_tests/fixtures/helpers/AppModeWidgetHelper.ts

@@ -0,0 +1,93 @@
+import type { Locator, Page } from '@playwright/test'
+
+import type { ComfyPage } from '../ComfyPage'
+
+/**
+ * Helper for interacting with widgets rendered in app mode (linear view).
+ *
+ * Widgets are located by their key (format: "nodeId:widgetName") via the
+ * `data-widget-key` attribute on each widget item.
+ */
+export class AppModeWidgetHelper {
+  constructor(private readonly comfyPage: ComfyPage) {}
+
+  private get page(): Page {
+    return this.comfyPage.page
+  }
+
+  private get container(): Locator {
+    return this.comfyPage.appMode.linearWidgets
+  }
+
+  /** Get a widget item container by its key (e.g. "6:text", "3:seed"). */
+  getWidgetItem(key: string): Locator {
+    return this.container.locator(`[data-widget-key="${key}"]`)
+  }
+
+  /** Fill a textarea widget (e.g. CLIP Text Encode prompt). */
+  async fillTextarea(key: string, value: string) {
+    const widget = this.getWidgetItem(key)
+    await widget.locator('textarea').fill(value)
+  }
+
+  /**
+   * Set a number input widget value (INT or FLOAT).
+   * Targets the last input inside the widget — this works for both
+   * ScrubableNumberInput (single input) and slider+InputNumber combos
+   * (last input is the editable number field).
+   */
+  async fillNumber(key: string, value: string) {
+    const widget = this.getWidgetItem(key)
+    const input = widget.locator('input').last()
+    await input.fill(value)
+    await input.press('Enter')
+  }
+
+  /** Fill a string text input widget (e.g. filename_prefix). */
+  async fillText(key: string, value: string) {
+    const widget = this.getWidgetItem(key)
+    await widget.locator('input').fill(value)
+  }
+
+  /** Select an option from a combo/select widget. */
+  async selectOption(key: string, optionName: string) {
+    const widget = this.getWidgetItem(key)
+    await widget.getByRole('combobox').click()
+    await this.page
+      .getByRole('option', { name: optionName, exact: true })
+      .click()
+  }
+
+  /**
+   * Intercept the /api/prompt POST, click Run, and return the prompt payload.
+   * Fulfills the route with a mock success response.
+   */
+  async runAndCapturePrompt(): Promise<
+    Record<string, { inputs: Record<string, unknown> }>
+  > {
+    let promptBody: Record<string, { inputs: Record<string, unknown> }> | null =
+      null
+    await this.page.route(
+      '**/api/prompt',
+      async (route, req) => {
+        promptBody = req.postDataJSON().prompt
+        await route.fulfill({
+          status: 200,
+          body: JSON.stringify({
+            prompt_id: 'test-id',
+            number: 1,
+            node_errors: {}
+          })
+        })
+      },
+      { times: 1 }
+    )
+
+    const responsePromise = this.page.waitForResponse('**/api/prompt')
+    await this.comfyPage.appMode.runButton.click()
+    await responsePromise
+
+    if (!promptBody) throw new Error('No prompt payload captured')
+    return promptBody
+  }
+}