Require concrete string header-name normalization outputs

Co-authored-by: Shri Sukhani <shrisukhani@users.noreply.github.com>

Author: cursoragent

hyperbrowser/header_utils.py

@@ -53,7 +53,7 @@ def normalize_headers(
             raise HyperbrowserError(effective_pair_error_message)
         try:
             normalized_key = key.strip()
-            if not isinstance(normalized_key, str):
+            if type(normalized_key) is not str:
                 raise TypeError("normalized header name must be a string")
         except HyperbrowserError:
             raise
@@ -104,7 +104,7 @@ def normalize_headers(
             raise HyperbrowserError("headers must not contain control characters")
         try:
             canonical_header_name = normalized_key.lower()
-            if not isinstance(canonical_header_name, str):
+            if type(canonical_header_name) is not str:
                 raise TypeError("canonical header name must be a string")
         except HyperbrowserError:
             raise

tests/test_header_utils.py

@@ -60,6 +60,15 @@ def lower(self):  # type: ignore[override]
         return object()
 
 
+class _StringSubclassStripResultHeaderName(str):
+    class _NormalizedKey(str):
+        pass
+
+    def strip(self, chars=None):  # type: ignore[override]
+        _ = chars
+        return self._NormalizedKey("X-Trace-Id")
+
+
 class _BrokenHeadersEnvString(str):
     def strip(self, chars=None):  # type: ignore[override]
         _ = chars
@@ -151,6 +160,18 @@ def test_normalize_headers_wraps_non_string_header_name_lower_results():
     assert exc_info.value.original_error is not None
 
 
+def test_normalize_headers_wraps_string_subclass_header_name_strip_results():
+    with pytest.raises(
+        HyperbrowserError, match="Failed to normalize header name"
+    ) as exc_info:
+        normalize_headers(
+            {_StringSubclassStripResultHeaderName("X-Trace-Id"): "trace-1"},
+            mapping_error_message="headers must be a mapping of string pairs",
+        )
+
+    assert isinstance(exc_info.value.original_error, TypeError)
+
+
 def test_normalize_headers_rejects_overly_long_header_names():
     long_header_name = "X-" + ("a" * 255)
     with pytest.raises(