fix(datastructures): extend_header_value only reads the first duplicate header entry, dropping the rest

`extend_header_value()` used `self.get(key)` to read the existing value
before joining in the new one. `MutableMapping.get()` is backed by
`__getitem__`, which returns only the FIRST raw header entry matching
`key` -- but a header can legitimately appear as multiple separate raw
entries in `self.headers` (see `add()`'s own docstring: "This method
keeps duplicates"), not just as one comma-joined value.

Concrete failure: add two raw "vary" entries ("accept", "accept-encoding"),
then extend_header_value("vary", "user-agent") -- the result silently
drops "accept-encoding" entirely, leaving only "accept,user-agent".

Fix: use `getall()` instead of `get()` to collect ALL duplicate entries,
flattening each entry's own comma-separated list (matching the existing
single-entry behavior) before joining in the new value. `getall()`
currently raises `KeyError` when the header isn't present at all (a
separate falsy-default bug also present in this method, which I've
already reported/fixed independently in PR #4924) -- catching that
`KeyError` here keeps this fix correct regardless of merge order
between the two PRs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: chuenchen309

litestar/datastructures/headers.py

@@ -188,9 +188,13 @@ def extend_header_value(self, key: str, value: str) -> None:
         Returns:
             None
         """
-        existing = self.get(key)
-        if existing is not None:
-            value = ",".join([*existing.split(","), value])
+        try:
+            existing_entries = self.getall(key)
+        except KeyError:
+            existing_entries = []
+        existing_values = [v for existing in existing_entries for v in existing.split(",")]
+        if existing_values:
+            value = ",".join([*existing_values, value])
         self[key] = value
 
     def __getitem__(self, key: str) -> str:

tests/unit/test_datastructures/test_headers.py

@@ -184,6 +184,17 @@ def test_mutable_scope_headers_from_tuple_extend_header_value_new_header(
     assert list(raw_headers_tuple) == [(b"foo", b"bar"), (b"bar", b"baz")]
 
 
+def test_mutable_scope_headers_extend_header_value_preserves_duplicate_entries(
+    mutable_headers: MutableScopeHeaders,
+) -> None:
+    # A header can legitimately appear as multiple separate raw entries (not just a
+    # single comma-joined value). extend_header_value must not silently drop any of
+    # them.
+    mutable_headers.add("foo", "baz")
+    mutable_headers.extend_header_value("foo", "qux")
+    assert mutable_headers.getall("foo") == ["bar,baz,qux"]
+
+
 def test_mutable_scope_headers_getitem(mutable_headers: MutableScopeHeaders, existing_headers_key: str) -> None:
     assert mutable_headers[existing_headers_key] == "bar"