[ajcvickers]

Adds a breaking change note to the EF Core 12 breaking changes page for dotnet/efcore#39630 (fixes dotnet/efcore#33452).

For owned JSON collections whose CLR property is annotated nullable, a stored NULL is now materialized as null even when the property has an initializer, and changes between null and empty are tracked and saved. The note describes the opt-out (SetIsNullable(false) or removing the annotation).

Draft until dotnet/efcore#39630 merges. Like the other open breaking change PRs, this adds ef-core-12.0/breaking-changes.md, so whichever merges second needs a trivial merge of the page.

🤖 Generated with Claude Code

[Copilot]

🟡 Changes recommended

The new EF Core 12.0 breaking-changes page is missing from entity-framework/toc.yml, so readers cannot discover it through the documentation navigation.

1 open issue

• Add the EF Core 12.0 page to the documentation TOC · New

What changed in this PR

Adds an EF Core 12 breaking-change note documenting how nullable owned JSON collections now preserve the distinction between stored NULL and empty collections, including materialization, change tracking, opt-out, and compiled-model guidance.

Changes:

• Documents old and new materialization and tracking behavior.
• Explains mitigation with nullable annotations or SetIsNullable(false).
• Includes related issue links and compiled-model regeneration guidance.

File	Description
entity-framework/​core/​what-is-new/​ef-core-12.0/​breaking-changes.md	Adds the EF Core 12 breaking-change documentation for nullable owned JSON collection properties.

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