Document Cosmos JSON breaking changes for EF Core 11

[Copilot]

EF Core 11 (dotnet/efcore#38550) replaces Newtonsoft.Json/JObject with System.Text.Json in the Cosmos materializer, removing __jObject and dropping unmapped property preservation.

Breaking changes documented (breaking-changes.md)

New High-impact changes section added before the existing Medium-impact section:

• __jObject shadow property removed — JObject no longer exists on entities; accessing it throws. Mitigation: use CosmosClient directly to read raw JSON.
• Unmapped properties no longer preserved — Extra JSON fields not in the EF model are silently dropped on round-trip. Mitigation: map all fields, or use CosmosClient.

New Low-impact entry added:

• Floating-point truncation — Query results cast to fixed-point types (int, decimal, etc.) are now truncated instead of rounded, fixing dotnet/efcore#38138.

What's new (whatsnew.md)

Added Modernized JSON serializer section to the Cosmos DB area noting the switch to System.Text.Json and linking to the breaking changes.

Provider docs (unstructured-data.md)

Updated the __jObject section to reflect that the property is gone in EF Core 11, with a link to the breaking change entry.