Fix seed data diffing when primary key column count changes

[AhmedSadek10]

• Check primary key column counts before comparing columns
• Add a test for expanding a primary key with seed data

Fixes #35116

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[Copilot]

Pull request overview

Fixes a migration scaffolding crash when diffing seed data for a table whose primary key column count changes between source and target models, by short-circuiting key mapping before per-column comparison.

Changes:

• In MigrationsModelDiffer.DiffData, check primary key column counts before iterating/comparing key columns to avoid out-of-range indexing.
• Add a regression test covering expanding a primary key when seed data is present.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
test/EFCore.Relational.Tests/Migrations/Internal/MigrationsModelDifferTest.cs	Adds a regression test for expanding a primary key with seed data.
src/EFCore.Relational/Migrations/Internal/MigrationsModelDiffer.cs	Prevents invalid key-column mapping when PK column counts differ during seed diffing.

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