Fix IsTableExcludedFromMigrations default for entity splitting.

[AndriySvyryd]

Fixes #38780

[Copilot]

Pull request overview

This PR fixes the default ExcludeFromMigrations behavior for entity-splitting mapping fragments so that a split table does not implicitly inherit the main table’s excluded-from-migrations setting unless it was explicitly configured on that fragment. This aligns migration diffing behavior with the per-table nature of entity splitting.

Changes:

• Update RelationalEntityTypeExtensions.IsTableExcludedFromMigrations(entityType, storeObject) so mapping fragments default to false when no explicit exclusion is configured.
• Add a regression test ensuring that removing entity splitting still produces DropTable/CreateTable operations for the split table even when the main table is excluded from migrations.

Reviewed changes

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

File	Description
test/EFCore.Relational.Tests/Migrations/Internal/MigrationsModelDifferTest.cs	Adds a regression test covering entity splitting removal when the main table is excluded from migrations.
src/EFCore.Relational/Extensions/RelationalEntityTypeExtensions.cs	Fixes mapping-fragment exclusion default to avoid inheriting the entity’s main table exclusion.

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