Fix complex type shadow discriminator incorrectly marked as modified on Update

[Copilot]

Fixes #38105

Regression in 10.0.3: DbSet.Update() on entities with nullable complex types having a discriminator throws InvalidOperationException: The property 'X.Discriminator' is defined as read-only after it has been saved, but its value has been modified or marked as modified.

Cause

PR #37394 (fix for #37337) changed SnapshotFactoryFactory.CreateSnapshotExpression to use default values for shadow properties on complex types. This was correct for ShadowValuesFactoryFactory (query materialization path, where complex type shadow property materialization is unsupported per #35613), but applied too broadly to all snapshot factories — including OriginalValuesFactoryFactory, which reads from IInternalEntry and can resolve shadow values.

The original values snapshot stored null for the discriminator instead of the actual value. Update() then detected a diff against the value-generator-assigned value and marked the read-only discriminator as modified.

Fix

• SnapshotFactoryFactory.cs: Narrowed the default-value fallback to only apply when the factory parameter is not IInternalEntry. Entry-based factories (OriginalValues, Relationship, Sidecar) can read shadow values from the entry's shadow storage directly.

if (!UseOldBehavior37337
    && propertyBase.DeclaringType is IComplexType
    && property.IsShadowProperty()
    && (parameter == null || !parameter.Type.IsAssignableTo(typeof(IInternalEntry))))

• AdHocComplexTypeQueryTestBase.cs: Added regression test Update_entity_with_nullable_complex_type_and_discriminator — seeds entity with nullable complex type + discriminator, loads it, calls context.Update(), asserts SaveChangesAsync() succeeds.

Existing #37337 test continues to pass (shadow values during query materialization still use defaults).

[AndriySvyryd]

Superseded by #38115