Streaming group eligibility requires the simple collation (query.getCollator() in eligibleForStreamingGroup). Support non-simple collations when the index and query collations match.

• StreamingAggStage already takes a collatorSlot; check that buildStreamingGroup passes it as hash $group does.
• computeSortsForScan already drops string sorts when the collators differ, so providedSorts() should enforce the match; verify it for object and compound keys.
• Add planner tests (matching, differing, simple vs. non-simple collators) and a case-insensitive jstest comparing results with hash $group.