Fix alter column type JSON to nVarchar

[atiq-bs23]

Fixes #38364

SQL Server's native json type cannot be implicitly converted to nvarchar in ALTER COLUMN. When the old column type is json, the generator now uses rename → add → CONVERT-copy → drop instead.

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[atiq-bs23]

@dotnet-policy-service agree

[Copilot]

Pull request overview

Fixes SQL Server migrations for ALTER COLUMN when converting a column from the native json type to nvarchar(...), which SQL Server doesn’t allow via implicit conversion. The SQL generator now emits a rename → add → CONVERT copy → drop flow instead of a direct ALTER COLUMN.

Changes:

• Updated SqlServerMigrationsSqlGenerator to special-case json→non-json type changes and generate a safe copy-based migration sequence.
• Added SQL generation tests validating the emitted DDL/DML for json→nvarchar(max) (nullable, non-nullable, idempotent) and that nvarchar→json still uses ALTER COLUMN.
• Added a functional test covering runtime execution for json→nvarchar(max) conversion (nullable case).

Reviewed changes

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

File	Description
src/EFCore.SqlServer/Migrations/SqlServerMigrationsSqlGenerator.cs	Implements rename/add/convert-copy/drop workaround for json→non-json ALTER COLUMN on SQL Server.
test/EFCore.SqlServer.FunctionalTests/Migrations/SqlServerMigrationsSqlGeneratorTest.cs	Adds SQL assertion tests for the new generation path and ensures nvarchar→json remains unchanged.
test/EFCore.SqlServer.FunctionalTests/Migrations/MigrationsSqlServerTest.cs	Adds an end-to-end functional test executing the json→nvarchar(max) workaround.

[atiq-bs23]

Hi @AndriySvyryd
Thanks for the thorough feedback. I have resolved all the comments:

• Added annotation copying (excluding identity) and Comment to the replacement column in AlterColumnFromJson, matching the drop+add path.

• Updated the integration test to declare an index on the converted column — used nvarchar(450) instead of nvarchar(max) since SQL Server doesn't support indexing nvarchar(max) as a key column.

Could you please take another look when you get a chance?

One note on running the tests locally: Version.Details.props references 11.0.0-preview.6.26313.102 which isn't publicly available yet, so the test runner fails to resolve the runtime. I was running tests against preview.5 locally as a workaround. This should not affect the PR itself but wanted to flag it in case it causes issues in CI.