Align HTTPBasic realm behavior with existing implementation

fastapi/security/http.py

@@ -298,10 +298,17 @@ class HTTPBearer(HTTPBase):
             ),
         ] = True,
     ):
-        self.model = HTTPBearerModel(bearerFormat=bearerFormat, description=description)
+        if realm is None:
+            raise ValueError(
+                "HTTP Basic authentication requires a realm as per RFC 7617"
+            )
+
+        self.model = HTTPBaseModel(scheme="basic", description=description)
         self.scheme_name = scheme_name or self.__class__.__name__
+        self.realm = realm
         self.auto_error = auto_error
 
+
     async def __call__(
         self, request: Request
     ) -> Optional[HTTPAuthorizationCredentials]:

tests/test_security_http_basic_optional.py

@@ -75,3 +75,5 @@ def test_openapi_schema():
             "securitySchemes": {"HTTPBasic": {"type": "http", "scheme": "basic"}}
         },
     }
+
+