Raise TypeError when setting StreamResponse.last_modified to an u…

…nsupported type (#10146)

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## What do these changes do?

closes #10143

## Are there changes in behavior for the user?

Currently this would silently fail and discard the new value.

<img width="685" alt="Screenshot 2024-12-08 at 12 32 49 PM"
src="https://github.com/user-attachments/assets/b0cf00c3-cd2e-4be6-af94-d993491bd296">

(cherry picked from commit a818e51)

CHANGES/10146.misc.rst

@@ -0,0 +1 @@
+Setting :attr:`aiohttp.web.StreamResponse.last_modified` to an unsupported type will now raise :exc:`TypeError` instead of silently failing -- by :user:`bdraco`.

aiohttp/web_response.py

@@ -370,6 +370,9 @@ def last_modified(
             )
         elif isinstance(value, str):
             self._headers[hdrs.LAST_MODIFIED] = value
+        else:
+            msg = f"Unsupported type for last_modified: {type(value).__name__}"  # type: ignore[unreachable]
+            raise TypeError(msg)
 
     @property
     def etag(self) -> Optional[ETag]:

tests/test_web_response.py

@@ -255,6 +255,13 @@ def test_last_modified_reset() -> None:
     assert resp.last_modified is None
 
 
+def test_last_modified_invalid_type() -> None:
+    resp = web.StreamResponse()
+
+    with pytest.raises(TypeError, match="Unsupported type for last_modified: object"):
+        resp.last_modified = object()  # type: ignore[assignment]
+
+
 @pytest.mark.parametrize(
     ["header_val", "expected"],
     [