refactor(tests): parametrize byte-type check and simplify assertions

Collapse the two duplicate test methods into one parametrized test,
move the isinstance assertion outside the Stream context manager, drop
the custom failure message, and use :class: RST roles in the changelog
entry — all per code-review feedback.

Author: CrepuscularIRIS

CHANGES/12404.bugfix.rst

@@ -1,3 +1,3 @@
 Fixed ``BodyPartReader.read()`` and ``BodyPartReader.read(decode=True)`` returning
-``bytearray`` instead of ``bytes``, violating the documented API contract.
+:class:`bytearray` instead of :class:`bytes`, violating the documented API contract.
 -- by :user:`CrepuscularIRIS`.

tests/test_multipart.py

@@ -185,28 +185,16 @@ async def test_read(self) -> None:
             assert b"Hello, world!" == result
             assert obj.at_eof()
 
-    async def test_read_returns_bytes_not_bytearray(self) -> None:
-        # Regression test for https://github.com/aio-libs/aiohttp/issues/12404
-        # read() must return bytes (not bytearray) to honour the documented API contract.
-        with Stream(b"Hello, world!\r\n--:") as stream:
-            d = CIMultiDictProxy[str](CIMultiDict())
-            obj = aiohttp.BodyPartReader(BOUNDARY, d, stream)
-            result = await obj.read()
-            assert isinstance(
-                result, bytes
-            ), f"Expected bytes, got {type(result).__name__}"
-
-    async def test_read_decode_returns_bytes_not_bytearray(self) -> None:
+    @pytest.mark.parametrize("kwargs", [{}, {"decode": True}])
+    async def test_read_returns_bytes_not_bytearray(
+        self, kwargs: dict[str, bool]
+    ) -> None:
         # Regression test for https://github.com/aio-libs/aiohttp/issues/12404
-        # read(decode=True) must return bytes (not bytearray) even when no
-        # Content-Transfer-Encoding / Content-Encoding header is present.
         with Stream(b"Hello, world!\r\n--:") as stream:
             d = CIMultiDictProxy[str](CIMultiDict())
             obj = aiohttp.BodyPartReader(BOUNDARY, d, stream)
-            result = await obj.read(decode=True)
-            assert isinstance(
-                result, bytes
-            ), f"Expected bytes, got {type(result).__name__}"
+            result = await obj.read(**kwargs)
+        assert isinstance(result, bytes)
 
     async def test_read_chunk_at_eof(self) -> None:
         with Stream(b"--:") as stream: