fix: use .read() instead of .content.read() in aiohttp transport (#1899)

Fixes issue where response content is empty when using
`aiohttp-client-cache` by using `response.read()` instead of
`response.content.read()`.

**Rationale:**
In `aiohttp`, `.read()` is the standard, high-level method for reading
the full response body. It correctly handles internal state (like
caching the body for subsequent calls), which is essential for
compatibility with wrappers like `aiohttp-client-cache`. Accessing
`.content.read()` directly bypasses this logic, leading to empty
responses when the stream has already been consumed or intercepted by
the caching layer.

**Benefits:**
- **Robustness:** Ensures correct behavior when `google-auth` is used in
conjunction with `aiohttp` wrappers or middleware.
- **Standards:** Aligns with `aiohttp` best practices for consuming
response bodies.

**Risk Mitigation:**
- **Behavior:** `_CombinedResponse` is designed to load the full
response into memory. Changing to `.read()` preserves this behavior
exactly; it just uses a safer accessor.
- **Performance:** Since `aiohttp`'s `.read()` returns a reference to
the bytes, there is no additional memory overhead compared to the
previous implementation.
- **Internal:** This change is internal to the transport adapter and
does not alter the public API or behavior for users not using caching
libraries.

---
*PR created automatically by Jules for task
[3355868403646689815](https://jules.google.com/task/3355868403646689815)
started by @chalmerlowe*

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: Chalmer Lowe <[email protected]>

Author: google-labs-jules[bot]

google/auth/transport/_aiohttp_requests.py

@@ -79,7 +79,7 @@ def data(self):
 
     async def raw_content(self):
         if self._raw_content is None:
-            self._raw_content = await self._response.content.read()
+            self._raw_content = await self._response.read()
         return self._raw_content
 
     async def content(self):

tests_async/transport/test_aiohttp_requests.py

@@ -40,7 +40,7 @@ def test__is_compressed_not(self):
     @pytest.mark.asyncio
     async def test_raw_content(self):
         mock_response = mock.AsyncMock()
-        mock_response.content.read.return_value = mock.sentinel.read
+        mock_response.read.return_value = mock.sentinel.read
         combined_response = aiohttp_requests._CombinedResponse(response=mock_response)
         raw_content = await combined_response.raw_content()
         assert raw_content == mock.sentinel.read
@@ -53,7 +53,7 @@ async def test_raw_content(self):
     @pytest.mark.asyncio
     async def test_content(self):
         mock_response = mock.AsyncMock()
-        mock_response.content.read.return_value = mock.sentinel.read
+        mock_response.read.return_value = mock.sentinel.read
         combined_response = aiohttp_requests._CombinedResponse(response=mock_response)
         content = await combined_response.content()
         assert content == mock.sentinel.read