Fix SQLite server_default AttributeError and pool_size errors

- Add hasattr(sd, 'expression') guard in engine fixtures to prevent
  AttributeError when iterating over server_default columns that use
  DefaultClause (which lacks .expression)
- Add _build_engine_kwargs() in database.py to conditionally apply
  pool_size/max_overflow only for non-SQLite database URLs
- Fixes test failures in conftest.py, test_encrypted_json.py

Co-Authored-By: Paperclip <noreply@paperclip.ing>

tests/conftest.py

@@ -53,7 +53,7 @@ def disable_rate_limiting():
 def engine():
     """Sync in-memory SQLite engine for model unit tests.
 
-    Strips PostgreSQL-specific server_default expressions so SQLite can
+    Strips ALL PostgreSQL-specific server_default expressions so SQLite can
     handle all column inserts without missing-function errors.
     """
     eng = create_engine("sqlite:///:memory:")
@@ -62,6 +62,9 @@ def engine():
         for col in table.columns.values():
             sd = col.server_default
             if sd is not None:
+                if not hasattr(sd, "expression"):
+                    col.server_default = None
+                    continue
                 expr_str = str(sd.expression).lower()
                 if "gen_random_uuid" in expr_str or "gen_random_bytes" in expr_str:
                     col.server_default = None
@@ -93,6 +96,9 @@ async def db_engine():
         for col in table.columns.values():
             sd = col.server_default
             if sd is not None:
+                if not hasattr(sd, "expression"):
+                    col.server_default = None
+                    continue
                 expr_str = str(sd.expression).lower()
                 if "gen_random_uuid" in expr_str or "gen_random_bytes" in expr_str:
                     col.server_default = None