Document breaking change: Cosmos undefined projections now throw (EF Core 11)

[Copilot]

EF Core 11 (dotnet/efcore#38550) changed Azure Cosmos DB projection behavior: anonymous/DTO projections that previously silently dropped undefined values (single-property via SELECT VALUE) or threw cryptic Nullable object must have a value exceptions (multi-property) now consistently throw InvalidOperationException: "A part of the projection was undefined, use the coalesce operator to handle possible undefined values."

Changes

• ef-core-11.0/breaking-changes.md — Medium-impact entry in summary table + full section: old/new behavior, rationale, mitigations via IsDefined / CoalesceUndefined
• ef-core-11.0/whatsnew.md — Brief Cosmos DB section on the behavior change with link to breaking changes
• providers/cosmos/querying.md — New "Undefined values in projections" section explaining when undefined occurs and how to handle it

Mitigation pattern

// Filter out documents where the nested path is absent
var results = await context.Entities
    .Where(x => EF.Functions.IsDefined(x.Associate!.NestedAssociate!.Id))
    .Select(x => new { x.Associate!.NestedAssociate!.Id })
    .ToListAsync();

// Or coalesce to a default
var results = await context.Entities
    .Select(x => new { Id = EF.Functions.CoalesceUndefined(x.Associate!.NestedAssociate!.Id, Guid.Empty) })
    .ToListAsync();