`unique_together` constraint not restored after field alteration when field has `unique=True`

[robberwick]

When a field has both unique=True AND participates in a unique_together constraint, altering that field (e.g., changing its type or nullability) causes the unique_together constraint to be dropped but not restored. Only the single-field unique constraint is restored.

In mssql/schema.py, the _alter_field method's restoration logic for unique_together is inside an else block that only executes when the field does NOT have unique=True.

Steps to Reproduce

1. Create a model with:
   • Field a with unique=True
   • Field b
   • unique_together = (('a', 'b'),) in Meta

class MyModel(models.Model):
    a = models.CharField(max_length=20, unique=True)
    b = models.CharField(max_length=20)

    class Meta:
        unique_together = (('a', 'b'),)

2. Create a migration that alters field a (e.g., changes max_length):

operations = [
    migrations.AlterField(
        model_name='mymodel',
        name='a',
        field=models.CharField(max_length=40, unique=True),  # changed from max_length=20
    ),
]

3. Run the migration
4. Check database constraints using introspection or SQL Server Management Studio

Expected Behavior

After altering field a, both constraints should exist in the database:

1. Single-field unique constraint on column a
2. Multi-field unique constraint on columns (a, b) from unique_together

Actual Behavior

Only the single-field unique constraint on a is restored. The unique_together constraint on (a, b) is lost permanently.

Django Versions Affected

All supported versions