Improve handling of null values in JSON properties mapped to primitive collections

[AndriySvyryd]

Fixes #38454

Changes materialization behavior when JSON contains the null token for primitive collections (both when mapped to a dedicated column and when nested inside a JSON document), aiming to return null for optional properties and throw a clearer, property-named exception for required ones.

Changes:

• Add a shaper-side fast path for primitive collections mapped to columns to detect JSON null before invoking CollectionToJsonStringConverter<>.
• Improve error reporting for required primitive collections nested in JSON by throwing RelationalStrings.NullValueInRequiredJsonProperty(...) instead of a reader/writer token-type exception.
• Add functional tests covering nullable/required primitive collections and converter ConvertsNulls behavior.

[Copilot]

Pull request overview

This PR addresses issue #38454 by improving materialization behavior when JSON contains the null token for primitive collections (both when mapped to a dedicated column and when nested inside a JSON document), aiming to return null for optional properties and throw a clearer, property-named exception for required ones.

Changes:

• Add a shaper-side fast path for primitive collections mapped to columns to detect JSON null before invoking CollectionToJsonStringConverter<>.
• Improve error reporting for required primitive collections nested in JSON by throwing RelationalStrings.NullValueInRequiredJsonProperty(...) instead of a reader/writer token-type exception.
• Add functional tests covering nullable/required primitive collections and converter ConvertsNulls behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
test/EFCore.SqlServer.FunctionalTests/Query/AdHocJsonQuerySqlServerTestBase.cs	Adds SQL Server functional tests for JSON null handling in primitive collections and converter null-conversion semantics.
src/EFCore.Relational/Query/RelationalShapedQueryCompilingExpressionVisitor.ShaperProcessingExpressionVisitor.cs	Updates the shaper expression generation to special-case primitive collection column materialization and to throw clearer errors for required JSON collections.
src/EFCore.Relational/Query/RelationalShapedQueryCompilingExpressionVisitor.ShaperProcessingExpressionVisitor.ClientMethods.cs	Introduces an internal helper used by the shaper to parse primitive collection JSON strings and handle the null token.