fix: add download retry to cache_audio_from_url matching cache_image_from_url (#3401)

PR #3323 added retry with exponential backoff to cache_image_from_url
but missed the sibling function cache_audio_from_url 18 lines below in
the same file. A single transient 429/5xx/timeout loses voice messages
while image downloads now survive them.

Apply the same retry pattern: 3 attempts with 1.5s exponential backoff,
immediate raise on non-retryable 4xx.

tests/gateway/test_media_download_retry.py

@@ -171,6 +171,170 @@ class TestCacheImageFromUrl:
         mock_sleep.assert_not_called()
 
 
+# ---------------------------------------------------------------------------
+# cache_audio_from_url (base.py)
+# ---------------------------------------------------------------------------
+
+class TestCacheAudioFromUrl:
+    """Tests for gateway.platforms.base.cache_audio_from_url"""
+
+    def test_success_on_first_attempt(self, tmp_path, monkeypatch):
+        """A clean 200 response caches the audio and returns a path."""
+        monkeypatch.setattr("gateway.platforms.base.AUDIO_CACHE_DIR", tmp_path / "audio")
+
+        fake_response = MagicMock()
+        fake_response.content = b"\x00\x01 fake audio"
+        fake_response.raise_for_status = MagicMock()
+
+        mock_client = AsyncMock()
+        mock_client.get = AsyncMock(return_value=fake_response)
+        mock_client.__aenter__ = AsyncMock(return_value=mock_client)
+        mock_client.__aexit__ = AsyncMock(return_value=False)
+
+        async def run():
+            with patch("httpx.AsyncClient", return_value=mock_client):
+                from gateway.platforms.base import cache_audio_from_url
+                return await cache_audio_from_url(
+                    "http://example.com/voice.ogg", ext=".ogg"
+                )
+
+        path = asyncio.run(run())
+        assert path.endswith(".ogg")
+        mock_client.get.assert_called_once()
+
+    def test_retries_on_timeout_then_succeeds(self, tmp_path, monkeypatch):
+        """A timeout on the first attempt is retried; second attempt succeeds."""
+        monkeypatch.setattr("gateway.platforms.base.AUDIO_CACHE_DIR", tmp_path / "audio")
+
+        fake_response = MagicMock()
+        fake_response.content = b"audio data"
+        fake_response.raise_for_status = MagicMock()
+
+        mock_client = AsyncMock()
+        mock_client.get = AsyncMock(
+            side_effect=[_make_timeout_error(), fake_response]
+        )
+        mock_client.__aenter__ = AsyncMock(return_value=mock_client)
+        mock_client.__aexit__ = AsyncMock(return_value=False)
+
+        mock_sleep = AsyncMock()
+
+        async def run():
+            with patch("httpx.AsyncClient", return_value=mock_client), \
+                 patch("asyncio.sleep", mock_sleep):
+                from gateway.platforms.base import cache_audio_from_url
+                return await cache_audio_from_url(
+                    "http://example.com/voice.ogg", ext=".ogg", retries=2
+                )
+
+        path = asyncio.run(run())
+        assert path.endswith(".ogg")
+        assert mock_client.get.call_count == 2
+        mock_sleep.assert_called_once()
+
+    def test_retries_on_429_then_succeeds(self, tmp_path, monkeypatch):
+        """A 429 response on the first attempt is retried; second attempt succeeds."""
+        monkeypatch.setattr("gateway.platforms.base.AUDIO_CACHE_DIR", tmp_path / "audio")
+
+        ok_response = MagicMock()
+        ok_response.content = b"audio data"
+        ok_response.raise_for_status = MagicMock()
+
+        mock_client = AsyncMock()
+        mock_client.get = AsyncMock(
+            side_effect=[_make_http_status_error(429), ok_response]
+        )
+        mock_client.__aenter__ = AsyncMock(return_value=mock_client)
+        mock_client.__aexit__ = AsyncMock(return_value=False)
+
+        async def run():
+            with patch("httpx.AsyncClient", return_value=mock_client), \
+                 patch("asyncio.sleep", new_callable=AsyncMock):
+                from gateway.platforms.base import cache_audio_from_url
+                return await cache_audio_from_url(
+                    "http://example.com/voice.ogg", ext=".ogg", retries=2
+                )
+
+        path = asyncio.run(run())
+        assert path.endswith(".ogg")
+        assert mock_client.get.call_count == 2
+
+    def test_retries_on_500_then_succeeds(self, tmp_path, monkeypatch):
+        """A 500 response on the first attempt is retried; second attempt succeeds."""
+        monkeypatch.setattr("gateway.platforms.base.AUDIO_CACHE_DIR", tmp_path / "audio")
+
+        ok_response = MagicMock()
+        ok_response.content = b"audio data"
+        ok_response.raise_for_status = MagicMock()
+
+        mock_client = AsyncMock()
+        mock_client.get = AsyncMock(
+            side_effect=[_make_http_status_error(500), ok_response]
+        )
+        mock_client.__aenter__ = AsyncMock(return_value=mock_client)
+        mock_client.__aexit__ = AsyncMock(return_value=False)
+
+        async def run():
+            with patch("httpx.AsyncClient", return_value=mock_client), \
+                 patch("asyncio.sleep", new_callable=AsyncMock):
+                from gateway.platforms.base import cache_audio_from_url
+                return await cache_audio_from_url(
+                    "http://example.com/voice.ogg", ext=".ogg", retries=2
+                )
+
+        path = asyncio.run(run())
+        assert path.endswith(".ogg")
+        assert mock_client.get.call_count == 2
+
+    def test_raises_after_max_retries_exhausted(self, tmp_path, monkeypatch):
+        """Timeout on every attempt raises after all retries are consumed."""
+        monkeypatch.setattr("gateway.platforms.base.AUDIO_CACHE_DIR", tmp_path / "audio")
+
+        mock_client = AsyncMock()
+        mock_client.get = AsyncMock(side_effect=_make_timeout_error())
+        mock_client.__aenter__ = AsyncMock(return_value=mock_client)
+        mock_client.__aexit__ = AsyncMock(return_value=False)
+
+        async def run():
+            with patch("httpx.AsyncClient", return_value=mock_client), \
+                 patch("asyncio.sleep", new_callable=AsyncMock):
+                from gateway.platforms.base import cache_audio_from_url
+                await cache_audio_from_url(
+                    "http://example.com/voice.ogg", ext=".ogg", retries=2
+                )
+
+        with pytest.raises(httpx.TimeoutException):
+            asyncio.run(run())
+
+        # 3 total calls: initial + 2 retries
+        assert mock_client.get.call_count == 3
+
+    def test_non_retryable_4xx_raises_immediately(self, tmp_path, monkeypatch):
+        """A 404 (non-retryable) is raised immediately without any retry."""
+        monkeypatch.setattr("gateway.platforms.base.AUDIO_CACHE_DIR", tmp_path / "audio")
+
+        mock_sleep = AsyncMock()
+        mock_client = AsyncMock()
+        mock_client.get = AsyncMock(side_effect=_make_http_status_error(404))
+        mock_client.__aenter__ = AsyncMock(return_value=mock_client)
+        mock_client.__aexit__ = AsyncMock(return_value=False)
+
+        async def run():
+            with patch("httpx.AsyncClient", return_value=mock_client), \
+                 patch("asyncio.sleep", mock_sleep):
+                from gateway.platforms.base import cache_audio_from_url
+                await cache_audio_from_url(
+                    "http://example.com/voice.ogg", ext=".ogg", retries=2
+                )
+
+        with pytest.raises(httpx.HTTPStatusError):
+            asyncio.run(run())
+
+        # Only 1 attempt, no sleep
+        assert mock_client.get.call_count == 1
+        mock_sleep.assert_not_called()
+
+
 # ---------------------------------------------------------------------------
 # Slack mock setup (mirrors existing test_slack.py approach)
 # ---------------------------------------------------------------------------