Document Cosmos DB id mapping changes in EF Core 9.0 and add SDK migration scripts

[Copilot]

Fixes #4883
Fixes #4821

EF Core 9.0 changed how the Azure Cosmos DB provider maps entity keys to the JSON id property and removed the discriminator from id by default. This PR adds documentation for those changes and provides Azure Cosmos DB SDK code samples to migrate existing documents.

modeling.md — IDs and keys section

• Explains that in EF 9.0 the entity key property IS the JSON id (no duplicate storage)
• Shows the resulting document format
• Adds a Shadow id subsection covering when EF falls back to a synthesized shadow id, and how to opt in explicitly via HasShadowId() / HasShadowIds()

breaking-changes.md — Migration code samples

Adds SDK-based migration code for each breaking change, for users who want to adopt the new defaults rather than revert:

Rename Discriminator → $type (non-breaking ReplaceItemAsync):

item["$type"] = item["Discriminator"]!.DeepClone();
item.Remove("Discriminator");
await container.ReplaceItemAsync(item, id, partitionKey);

Strip discriminator prefix from id (requires delete + re-create since Cosmos DB prohibits id updates):

var newId = oldId[(oldId.IndexOf('|') + 1)..];
item["id"] = JsonValue.Create(newId);
await container.CreateItemAsync(item, partitionKey);
await container.DeleteItemAsync<JsonObject>(oldId, partitionKey);

Remove redundant key property (now that id and the key property are the same):

item.Remove("Id");
await container.ReplaceItemAsync(item, id, partitionKey);

Each sample includes notes on partition key configuration and idempotency considerations for the delete+re-create pattern.

[Copilot]

Pull request overview

This PR updates EF Core 9.0 Cosmos DB documentation to explain the new id/key mapping behavior and adds Azure Cosmos DB SDK migration snippets to help users update existing documents to the new defaults.

Changes:

• Document EF Core 9.0’s Cosmos id mapping (key maps directly to JSON id) and introduce guidance for shadow id usage.
• Add Cosmos SDK migration examples for renaming Discriminator → $type, rewriting id values, and removing redundant key properties.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
entity-framework/core/what-is-new/ef-core-9.0/breaking-changes.md	Adds Cosmos SDK-based migration snippets for EF Core 9.0 Cosmos breaking changes.
entity-framework/core/providers/cosmos/modeling.md	Updates Cosmos modeling docs to describe EF Core 9.0 key/id mapping and shadow id behavior.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.