Distinguish between truncated and excess content in response

`HTTPResponse._raw_read` raises `IncompleteRead` if the content length does
not match the expected content length. For malformed responses (e.g. 204
response with content) the re-raised `ProtocolError` was a bit too unclear
about the unexpected excess content being the reason for the exception.

With this change, the exception points out, that the client is not dealing
with a connection error but a protocol violation.

Closes urllib3#3261

changelog/3261.misc.rst

@@ -0,0 +1 @@
+Made re-raised ProtocolError more verbose regarding corner cases.

src/urllib3/response.py

@@ -749,8 +749,18 @@ def _error_catcher(self) -> typing.Generator[None, None, None]:
 
                 raise ReadTimeoutError(self._pool, None, "Read timed out.") from e  # type: ignore[arg-type]
 
+            except IncompleteRead as e:
+                if (
+                    e.expected is not None
+                    and e.partial is not None
+                    and int(e.expected) == -int(e.partial)
+                ):
+                    arg = f"Protocol violation: Response may not contain content: {e!r}"
+                else:
+                    arg = f"Connection broken: {e!r}"
+                raise ProtocolError(arg, e) from e
+
             except (HTTPException, OSError) as e:
-                # This includes IncompleteRead.
                 raise ProtocolError(f"Connection broken: {e!r}", e) from e
 
             # If no exception is thrown, we should avoid cleaning up

test/test_response.py

@@ -1504,6 +1504,33 @@ def make_bad_mac_fp() -> typing.Generator[BytesIO, None, None]:
                 resp.read()
             assert e.value.args[0] == mac_error
 
+    def test_unexpected_body(self) -> None:
+        with pytest.raises(ProtocolError) as excinfo:
+            fp = BytesIO(b"12345")
+            headers = {"content-length": "5"}
+            resp = HTTPResponse(fp, status=204, headers=headers)
+            resp.read(16)
+        assert "Protocol violation: Response may not contain content" in str(
+            excinfo.value
+        )
+
+        with pytest.raises(ProtocolError):
+            fp = BytesIO(b"12345")
+            headers = {"content-length": "0"}
+            resp = HTTPResponse(fp, status=204, headers=headers)
+            resp.read(16)
+        assert "Protocol violation: Response may not contain content" in str(
+            excinfo.value
+        )
+
+        with pytest.raises(ProtocolError):
+            fp = BytesIO(b"12345")
+            resp = HTTPResponse(fp, status=204)
+            resp.read(16)
+        assert "Protocol violation: Response may not contain content" in str(
+            excinfo.value
+        )
+
 
 class MockChunkedEncodingResponse:
     def __init__(self, content: list[bytes]) -> None: