ref(issues): Move group stats endpoint and tests to issues folder (#97317)

## Summary
- Move `src/sentry/api/endpoints/group_stats.py` to
`src/sentry/issues/endpoints/`
- Move `tests/sentry/api/endpoints/test_group_stats.py` to
`tests/sentry/issues/endpoints/`
- Update import references in `src/sentry/api/urls.py`

## Test plan
- [x] Verify endpoint file moved successfully
- [x] Verify test file moved successfully  
- [x] Verify import paths updated correctly
- [x] Pre-commit hooks pass


Part of
https://linear.app/getsentry/issue/RTC-1110/move-issues-endpoints-into-issues-app

---------

Co-authored-by: getsantry[bot] <66042841+getsantry[bot]@users.noreply.github.com>

Author: JoshFerge

src/sentry/api/urls.py

@@ -230,6 +230,7 @@
 )
 from sentry.issues.endpoints.browser_reporting_collector import BrowserReportingCollectorEndpoint
 from sentry.issues.endpoints.group_reprocessing import GroupReprocessingEndpoint
+from sentry.issues.endpoints.group_stats import GroupStatsEndpoint
 from sentry.issues.endpoints.organization_group_search_view_starred_order import (
     OrganizationGroupSearchViewStarredOrderEndpoint,
 )
@@ -503,7 +504,6 @@
 from .endpoints.group_current_release import GroupCurrentReleaseEndpoint
 from .endpoints.group_external_issue_details import GroupExternalIssueDetailsEndpoint
 from .endpoints.group_first_last_release import GroupFirstLastReleaseEndpoint
-from .endpoints.group_stats import GroupStatsEndpoint
 from .endpoints.group_tagkey_details import GroupTagKeyDetailsEndpoint
 from .endpoints.group_tagkey_values import GroupTagKeyValuesEndpoint
 from .endpoints.group_tags import GroupTagsEndpoint