refactor: use asyncio.TaskGroup in the error snapshotter (#2321)

Converts the error snapshotter's concurrent HTML and screenshot saves to
`asyncio.TaskGroup`. Its caller catches and logs any `Exception`, so the
`ExceptionGroup` doesn't change behavior.

The other places from the issue keep their current code:

- `TaskGroup` wraps failures in an `ExceptionGroup`, which defeats the
crawler's type-based error handling (retries, error statistics) and
users' `except` clauses. That rules out the robots.txt skips,
`BrowserPool.new_page_with_each_plugin` and `infinite_scroll`, where the
gain is small.
- `ThrottlingRequestManager` reads are side-effect-free, so a task group
adds code without changing anything. `drop`, `purge` and the sub-manager
opening must let every call finish.
- `LocalEventManager` runs its readings in `to_thread`, which
cancellation can't stop.
- The autoscaled pool would start cancelling healthy workers when one
fails.
- The event manager, RQ background batches, `RecurringTask` and the
sitemap loader own tasks that outlive the call that starts them.
`_merge_async_generators` is an async generator.

Closes: #1722

*✍️ Drafted by Claude Code*

Author: vdusek

src/crawlee/statistics/_error_snapshotter.py

@@ -44,19 +44,13 @@ async def capture_snapshot(
         if snapshot := await context.get_snapshot():
             kvs = await KeyValueStore.open(name=self._kvs_name)
             snapshot_base_name = self._get_snapshot_base_name(error_message, file_and_line)
-            snapshot_save_tasks = list[asyncio.Task]()
 
-            if snapshot.html:
-                snapshot_save_tasks.append(
-                    asyncio.create_task(self._save_html(kvs, snapshot.html, base_name=snapshot_base_name))
-                )
+            async with asyncio.TaskGroup() as tg:
+                if snapshot.html:
+                    tg.create_task(self._save_html(kvs, snapshot.html, base_name=snapshot_base_name))
 
-            if snapshot.screenshot:
-                snapshot_save_tasks.append(
-                    asyncio.create_task(self._save_screenshot(kvs, snapshot.screenshot, base_name=snapshot_base_name))
-                )
-
-            await asyncio.gather(*snapshot_save_tasks)
+                if snapshot.screenshot:
+                    tg.create_task(self._save_screenshot(kvs, snapshot.screenshot, base_name=snapshot_base_name))
 
     async def _save_html(self, kvs: KeyValueStore, html: str, base_name: str) -> None:
         file_name = f'{base_name}.html'