Scope SearchHelper autocomplete helpers to Current.organization
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## What
Update the `SearchHelper` autocomplete methods to scope results to `Current.organization` when organization isolation is enabled.
## Why
The following methods fetch groups and projects instance-wide with no organization boundary:
- **`groups_autocomplete`** — calls `current_user.search_on_authorized_groups(term)` with no organization filter.
- **`projects_autocomplete`** — calls `current_user.authorized_projects` with no organization filter.
`recent_issues_autocomplete` and `recent_merge_requests_autocomplete` delegate to `Gitlab::Search::RecentIssues` and `Gitlab::Search::RecentMergeRequests` respectively — those are covered in a separate issue.
## Implementation notes
- `groups_autocomplete`: chain `.in_organization(Current.organization)` on the group relation when isolation is enabled.
- `projects_autocomplete`: pass `organization: Current.organization` to the underlying `ProjectsFinder` or chain `.in_organization` on `current_user.authorized_projects` when isolation is enabled. Confirm which approach is cleaner given the current call.
- All changes must be gated on organization isolation being active.
## Files
- `app/helpers/search_helper.rb` — `#groups_autocomplete`, `#projects_autocomplete`
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