Update tests to reflect correct behavior for form/header default values

The previous test expectations were based on buggy behavior where default
values were incorrectly included in validation error input messages. After
fixing issue #13399 to preserve model_fields_set correctly, validation
errors now correctly show only the actual input received, not pre-filled
defaults.

Changes:
- test_forms_single_model.py: Updated test_no_data to expect empty input dict
- test_tutorial001.py: Removed x_tag default from validation error input
- test_tutorial002.py: Removed x_tag default from validation error input
- test_tutorial003.py: Removed x_tag default from both invalid tests

These changes ensure tests validate the correct behavior where defaults
are not pre-filled in the input dict passed to Pydantic, which allows
model_fields_set to accurately track which fields were explicitly provided.

tests/test_forms_single_model.py

@@ -99,13 +99,13 @@ def test_no_data():
                 "type": "missing",
                 "loc": ["body", "username"],
                 "msg": "Field required",
-                "input": {"tags": ["foo", "bar"], "with": "nothing"},
+                "input": {},
             },
             {
                 "type": "missing",
                 "loc": ["body", "lastname"],
                 "msg": "Field required",
-                "input": {"tags": ["foo", "bar"], "with": "nothing"},
+                "input": {},
             },
         ]
     }

tests/test_tutorial/test_header_param_models/test_tutorial001.py

@@ -67,7 +67,6 @@ def test_header_param_model_invalid(client: TestClient):
                     "loc": ["header", "save_data"],
                     "msg": "Field required",
                     "input": {
-                        "x_tag": [],
                         "host": "testserver",
                         "accept": "*/*",
                         "accept-encoding": "gzip, deflate",

tests/test_tutorial/test_header_param_models/test_tutorial002.py

@@ -67,7 +67,7 @@ def test_header_param_model_invalid(client: TestClient):
                     "type": "missing",
                     "loc": ["header", "save_data"],
                     "msg": "Field required",
-                    "input": {"x_tag": [], "host": "testserver"},
+                    "input": {"host": "testserver"},
                 }
             ]
         }

tests/test_tutorial/test_header_param_models/test_tutorial003.py

@@ -66,7 +66,6 @@ def test_header_param_model_no_underscore(client: TestClient):
                     "input": {
                         "host": "testserver",
                         "traceparent": "123",
-                        "x_tag": [],
                         "accept": "*/*",
                         "accept-encoding": "gzip, deflate",
                         "connection": "keep-alive",
@@ -104,7 +103,6 @@ def test_header_param_model_invalid(client: TestClient):
                     "loc": ["header", "save_data"],
                     "msg": "Field required",
                     "input": {
-                        "x_tag": [],
                         "host": "testserver",
                         "accept": "*/*",
                         "accept-encoding": "gzip, deflate",