fix: replace mutable default arg with None and add encoding='utf-8' to open() calls

- fastapi/_compat/v2.py: Replace mutable default argument
  with  in ModelField.validate() to avoid
  shared state across calls (B006/W0102)

- scripts/add_latest_release_date.py: Add encoding='utf-8' to both open() calls
  to ensure consistent file reading/writing across platforms (W1514)

fastapi/_compat/v2.py

@@ -173,7 +173,7 @@ class ModelField:
     def validate(
         self,
         value: Any,
-        values: dict[str, Any] = {},  # noqa: B006
+        values: dict[str, Any] | None = None,
         *,
         loc: tuple[int | str, ...] = (),
     ) -> tuple[Any, list[dict[str, Any]]]:

scripts/add_latest_release_date.py

@@ -9,7 +9,7 @@ RELEASE_HEADER_PATTERN = re.compile(r"^## (\d+\.\d+\.\d+)\s*(\(.*\))?\s*$")
 
 
 def main() -> None:
-    with open(RELEASE_NOTES_FILE) as f:
+    with open(RELEASE_NOTES_FILE, encoding="utf-8") as f:
         lines = f.readlines()
 
     for i, line in enumerate(lines):
@@ -28,7 +28,7 @@ def main() -> None:
         lines[i] = f"## {version} ({today})\n"
         print(f"Added date: {version} ({today})")
 
-        with open(RELEASE_NOTES_FILE, "w") as f:
+        with open(RELEASE_NOTES_FILE, "w", encoding="utf-8") as f:
             f.writelines(lines)
         sys.exit(0)