fix: convert all FastAPI lifespan context managers to async and resolve MRO issue

- Change SoftDeleteMixin to not inherit from SQLModel (avoids MRO conflict)
- Update all FastAPI tutorial files to use @asynccontextmanager instead of @contextmanager
  for lifespan (FastAPI 0.111+ requirement)
- Add missing imports (HTTPException, Depends, Query) where needed
- Fix test that called deprecated on_startup() method

Author: letsdeepchat

docs_src/tutorial/fastapi/update/tutorial002_py310.py

@@ -45,17 +45,13 @@ def hash_password(password: str) -> str:
     return f"not really hashed {password} hehehe"
 
 
-def get_session():
-    with Session(engine) as session:
-        yield session
-
-
-app = FastAPI()
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    create_db_and_tables()
+    yield
 
 
-@app.on_event("startup")
-def on_startup():
-    create_db_and_tables()
+app = FastAPI(lifespan=lifespan)
 
 
 @app.post("/heroes/", response_model=HeroPublic)