docs: fix inline code formatting and add tested example for Required, can be None section

docs/en/docs/tutorial/query-params-str-validations.md

@@ -257,23 +257,12 @@ For example, you might try:
 q: Annotated[str | None, Query(min_length=3)] = ...
 ```
 
-But this will still expect a string value, and if the client omits q or tries to send q=None, FastAPI will raise a validation error.
-In other words, None is not an acceptable runtime value for query parameters — only strings are.
+But this will still expect a **string** value, and if the client omits `q` or tries to send `q=None`, FastAPI will raise a validation error.  
+In other words, `None` is not an acceptable runtime value for query parameters — only strings are.
 
-If you want to accept special values (like "None" or an empty string) and interpret them as None in your application, you can handle them manually in your function:
+If you want to accept special values (like `"None"` or an empty string) and interpret them as `None` in your application, you can handle them manually in your function:
 
-```Python
-from fastapi import FastAPI, Query
-from typing import Annotated
-
-app = FastAPI()
-
-@app.get("/items/")
-async def read_items(q: Annotated[str | None, Query()] = ...):
-    if q in ("None", "", "null"):
-        q = None
-    return {"q": q}
-```
+{* ../../docs_src/query_params_str_validations/tutorial006d_an_py310.py hl[9] *}
 
 ## Query parameter list / multiple values { #query-parameter-list-multiple-values }
 

docs_src/query_params_str_validations/tutorial006d_an_py310.py

@@ -0,0 +1,10 @@
+from fastapi import FastAPI, Query
+from typing import Annotated
+
+app = FastAPI()
+
+@app.get("/items/")
+async def read_items(q: Annotated[str | None, Query()] = ...):
+    if q in ("None", "", "null"):
+        q = None
+    return {"q": q}

tests/test_tutorial/test_query_params_str_validations/test_tutorial006d_an_py310.py

@@ -0,0 +1,9 @@
+from fastapi.testclient import TestClient
+from docs_src.query_params_str_validations.tutorial006d_an_py310 import app
+
+client = TestClient(app)
+
+def test_read_items():
+    response = client.get("/items/", params={"q": "None"})
+    assert response.status_code == 200
+    assert response.json() == {"q": None}