refactor: extract assetPath as standalone pure function (#10651)

## Summary

Extract `assetPath` from a `ComfyPage` method to a standalone pure
function, removing unnecessary coupling to the page object.

## Changes

- **What**: Moved `assetPath` to
`browser_tests/fixtures/utils/paths.ts`. `DragDropHelper` and
`WorkflowHelper` import it directly instead of receiving it via
`ComfyPage`. `ComfyPage.assetPath` kept as thin delegate for backward
compat.

## Review Focus

Structural-only refactor — no behavioral changes. The function was
already pure (no `this`/`page` usage).

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10651-refactor-extract-assetPath-as-standalone-pure-function-3316d73d365081c0b0e0ce6dde57ef8e)
by [Unito](https://www.unito.io)

browser_tests/fixtures/helpers/DragDropHelper.ts

@@ -4,12 +4,10 @@ import type { Page } from '@playwright/test'
 
 import type { Position } from '../types'
 import { getMimeType } from './mimeTypeUtil'
+import { assetPath } from '../utils/paths'
 
 export class DragDropHelper {
-  constructor(
-    private readonly page: Page,
-    private readonly assetPath: (fileName: string) => string
-  ) {}
+  constructor(private readonly page: Page) {}
 
   private async nextFrame(): Promise<void> {
     await this.page.evaluate(() => {
@@ -49,7 +47,7 @@ export class DragDropHelper {
     } = { dropPosition, preserveNativePropagation }
 
     if (fileName) {
-      const filePath = this.assetPath(fileName)
+      const filePath = assetPath(fileName)
       const buffer = readFileSync(filePath)
 
       evaluateParams.fileName = fileName