fix: gate Matrix adapter path on media_files presence

Text-only Matrix sends should continue using the lightweight _send_matrix()
HTTP helper (~100ms). Only route through the heavy MatrixAdapter (full sync +
E2EE setup) when media files are present. Adds test verifying text-only
messages don't take the adapter path.

tests/tools/test_send_message_tool.py

@@ -622,6 +622,25 @@ class TestSendToPlatformChunking:
         finally:
             doc_path.unlink(missing_ok=True)
 
+    def test_matrix_text_only_uses_lightweight_path(self):
+        """Text-only Matrix sends should NOT go through the heavy adapter path."""
+        helper = AsyncMock()
+        lightweight = AsyncMock(return_value={"success": True, "platform": "matrix", "chat_id": "!room:ex.com", "message_id": "$txt"})
+        with patch("tools.send_message_tool._send_matrix_via_adapter", helper), \
+             patch("tools.send_message_tool._send_matrix", lightweight):
+            result = asyncio.run(
+                _send_to_platform(
+                    Platform.MATRIX,
+                    SimpleNamespace(enabled=True, token="tok", extra={"homeserver": "https://matrix.example.com"}),
+                    "!room:ex.com",
+                    "just text, no files",
+                )
+            )
+
+        assert result["success"] is True
+        helper.assert_not_awaited()
+        lightweight.assert_awaited_once()
+
     def test_send_matrix_via_adapter_sends_document(self, tmp_path):
         file_path = tmp_path / "report.pdf"
         file_path.write_bytes(b"%PDF-1.4 test")