Upgrade Pydantic to v2.12.0 and remove hack for Python 3.13 (#26481)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/config/utils.py

@@ -111,17 +111,7 @@ def get_attr_docs(cls: type[Any]) -> dict[str, str]:
     https://davidism.com/mit-license/
     """
 
-    try:
-        cls_node = ast.parse(textwrap.dedent(inspect.getsource(cls))).body[0]
-    except (OSError, KeyError, TypeError):
-        # HACK: Python 3.13+ workaround - set missing __firstlineno__
-        # Workaround can be removed after we upgrade to pydantic==2.12.0
-        with open(inspect.getfile(cls)) as f:
-            for i, line in enumerate(f):
-                if f"class {cls.__name__}" in line and ":" in line:
-                    cls.__firstlineno__ = i + 1
-                    break
-        cls_node = ast.parse(textwrap.dedent(inspect.getsource(cls))).body[0]
+    cls_node = ast.parse(textwrap.dedent(inspect.getsource(cls))).body[0]
 
     if not isinstance(cls_node, ast.ClassDef):
         raise TypeError("Given object was not a class.")