refactor: encapsulate error extraction in TaskItemImpl getters (#7650)

## Summary
- Add `errorMessage` and `executionError` getters to `TaskItemImpl` that
extract error info from status messages
- Update `useJobErrorReporting` composable to use these getters instead
of standalone function
- Remove the standalone `extractExecutionError` function

This encapsulates error extraction within `TaskItemImpl`, preparing for
the Jobs API migration where the underlying data format will change but
the getter interface will remain stable.

## Test plan
- [x] All existing tests pass
- [x] New tests added for `TaskItemImpl.errorMessage` and
`TaskItemImpl.executionError` getters
- [x] TypeScript, lint, and knip checks pass

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

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7650-refactor-encapsulate-error-extraction-in-TaskItemImpl-getters-2ce6d73d365081caae33dcc7e1e07720)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Christian Byrne <cbyrne@comfy.org>

browser_tests/fixtures/ComfyPage.ts

@@ -21,7 +21,6 @@ import {
 import { Topbar } from './components/Topbar'
 import type { Position, Size } from './types'
 import { NodeReference, SubgraphSlotReference } from './utils/litegraphUtils'
-import TaskHistory from './utils/taskHistory'
 
 dotenv.config()
 
@@ -146,8 +145,6 @@ class ConfirmDialog {
 }
 
 export class ComfyPage {
-  private _history: TaskHistory | null = null
-
   public readonly url: string
   // All canvas position operations are based on default view of canvas.
   public readonly canvas: Locator
@@ -301,11 +298,6 @@ export class ComfyPage {
     }
   }
 
-  setupHistory(): TaskHistory {
-    this._history ??= new TaskHistory(this)
-    return this._history
-  }
-
   async setup({
     clearStorage = true,
     mockReleases = true