Split service_instances_spec.rb for better parallelization

Split large 4,605-line request spec into 5 focused files:
- service_instances_show_spec.rb (855 lines - GET endpoints)
- service_instances_create_spec.rb (873 lines - POST)
- service_instances_update_spec.rb (1,280 lines - PATCH)
- service_instances_delete_spec.rb (788 lines - DELETE)
- service_instances_sharing_spec.rb (704 lines - sharing relationships)

Created shared_contexts/service_instances_context.rb with:
- Common let blocks (user, org, space, annotations)
- Helper methods (create_managed_json, create_user_provided_json, etc.)

Benefits:
- Better parallelization: 4,605 lines on 1 worker → 5 files across workers
- Estimated savings: 3-4 minutes → 1-1.5 minutes = ~2.5 minutes
- Improved maintainability: smaller, focused files
- No code duplication: shared setup in one place

Verification:
- All 510 examples pass (0 failures)
- Same execution time as original
- Clean refactor with shared_context pattern

Original file renamed to service_instances_spec.rb.original as backup.

Week 1 Task 1: Split large files (1 of 3 complete)

Author: johha