Cherry-pick case-insensitive in fix for release 6.5.4

docs/releases.rst

@@ -4,6 +4,7 @@ Release notes
 .. toctree::
    :maxdepth: 2
 
+   releases/v6.5.4
    releases/v6.5.3
    releases/v6.5.2
    releases/v6.5.1

docs/releases/v6.5.4.rst

@@ -0,0 +1,13 @@
+What's new in Tornado 6.5.4
+===========================
+
+Dec 15, 2025
+------------
+
+Bug fixes
+~~~~~~~~~
+
+- The ``in`` operator for ``HTTPHeaders`` was incorrectly case-sensitive, causing
+  lookups to fail for headers with different casing than the original header name.
+  This was a regression in version 6.5.3 and has been fixed to restore the intended
+  case-insensitive behavior from version 6.5.2 and earlier.

tornado/__init__.py

@@ -22,8 +22,8 @@
 # is zero for an official release, positive for a development branch,
 # or negative for a release candidate or beta (after the base version
 # number has been incremented)
-version = "6.5.3"
-version_info = (6, 5, 3, 0)
+version = "6.5.4"
+version_info = (6, 5, 4, 0)
 
 import importlib
 import typing

tornado/httputil.py

@@ -345,7 +345,8 @@ def __contains__(self, name: object) -> bool:
         # in __getitem__ when it's not needed.
         if not isinstance(name, str):
             return False
-        return name in self._as_list
+        norm_name = _normalize_header(name)
+        return norm_name in self._as_list
 
     def __getitem__(self, name: str) -> str:
         header = _normalize_header(name)

tornado/test/httputil_test.py

@@ -329,6 +329,9 @@ def test_multi_line(self):
             sorted(list(headers.get_all())),
             [("Asdf", "qwer zxcv"), ("Foo", "bar baz"), ("Foo", "even more lines")],
         )
+        # Verify case insensitivity in-operator
+        self.assertTrue("asdf" in headers)
+        self.assertTrue("Asdf" in headers)
 
     def test_continuation(self):
         data = "Foo: bar\r\n\tasdf"