fix(postgres): declare JSONB expression formatter protocol

Ensure the PostgreSQL JSONB enhanced protocol declares the public
formatter implemented by its mixin so protocol conformance checks pass.

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

Author: vistart

src/rhosocial/activerecord/backend/impl/postgres/protocols/types/jsonb_enhanced.py

@@ -38,6 +38,10 @@ class PostgresJSONBEnhancedSupport(Protocol):
     - Numeric infinity in JSONB: PostgreSQL 17+
     """
 
+    def format_json_expression(self, expr):
+        """Format a PostgreSQL JSON expression."""
+        ...
+
     def supports_json_type(self) -> bool:
         """Whether JSON data type is supported.