fix: remove transaction commit from duplicate guild ID test

Database integrity constraints are enforced during flush(),
not just on commit(). Remove unnecessary commit() and rollback()
calls - the fixture handles transaction cleanup.

Fixes test: TestDatabaseIntegrity::test_duplicate_guild_id_rejected

Author: JacobCoffee

tests/integration/test_api_endpoints.py

@@ -526,7 +526,6 @@ async def test_duplicate_guild_id_rejected(
         guild1 = Guild(guild_id=5555, guild_name="First Guild")
         db_session.add(guild1)
         await db_session.flush()
-        await db_session.commit()
 
         # Try to create duplicate - should raise integrity error
         from sqlalchemy.exc import IntegrityError
@@ -537,8 +536,6 @@ async def test_duplicate_guild_id_rejected(
         with pytest.raises(IntegrityError):
             await db_session.flush()
 
-        await db_session.rollback()
-
     async def test_foreign_key_constraint_enforced(
         self,
         db_session: AsyncSession,