Track optional entity splitting fragments using linking FKs

- [ ] Create linking FKs eagerly when configuring entity splitting (allowing non-identifying FKs(https://github.com/dotnet/efcore/issues/36251))
- [ ] Store optionality in table-specific `IsRequiredDependent` configuration for the linking FK
- [ ] Change the query code to use linking FKs instead of fragments

Start by tracing entity-splitting configuration and the query code that currently uses fragments. Verify how linking foreign keys are configured, how table-specific optionality is stored, and whether queries use those linking FKs; the work is done when all three checklist items are implemented and optional entity-splitting fragments are tracked correctly.

Written by the indexing model from the issue text.