Emit oldStored based on the old column in CSharpMigrationOperationGen…

[ilkertskn]

…erator

• The oldStored: argument of a generated AlterColumn call was gated on operation.IsStored (the new column) while emitting operation.OldColumn.IsStored, so altering a column away from a stored computed column dropped oldStored: from the generated (and reverse) migration
• Gate on operation.OldColumn.IsStored to match the value being written
• Add a test covering alter from a stored computed column to a non-computed column

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        Summary of the changes
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        Fixes #bugnumber

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[ilkertskn]

@dotnet-policy-service agree

[Copilot]

Pull request overview

Fixes C# migration code generation for AlterColumn so oldStored: is emitted based on operation.OldColumn.IsStored (the value actually being written), preventing loss of oldStored: when altering away from a stored computed column.

Changes:

• Gate emission of oldStored: on operation.OldColumn.IsStored instead of operation.IsStored.
• Add a regression test covering altering from a stored computed column to a non-computed column and ensuring oldStored: is preserved.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/EFCore.Design/Migrations/Design/CSharpMigrationOperationGenerator.cs	Fixes the conditional that controls whether oldStored: is emitted for AlterColumn.
test/EFCore.Design.Tests/Migrations/Design/CSharpMigrationOperationGeneratorTest.cs	Adds a regression test ensuring oldStored: is generated when removing a stored computed column.