refactor(sqlserver): validate COLLATE names via dialect hook

Return the validated SQL Server collation SQL token from validate_collation_name
so COLLATE formatting supports DATABASE_DEFAULT through one hook.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: vistart

src/rhosocial/activerecord/backend/impl/sqlserver/dialect.py

@@ -277,8 +277,8 @@ def supports_collate_expression(self) -> bool:
         """SQL Server supports expression-level COLLATE."""
         return True
 
-    def format_collation_name(self, collation) -> str:
-        """Format SQL Server collation names as validated bare tokens."""
+    def validate_collation_name(self, collation) -> str:
+        """Validate SQL Server collation names and return their SQL representation."""
         if collation.schema is not None:
             raise UnsupportedFeatureError(self.name, "schema-qualified COLLATE")
         if collation.keyword is not None and collation.keyword != "DATABASE_DEFAULT":