fix(scouts): fetch single Gmail message instead of bulk in fetch_content

Replace bulk GmailClient.fetch_messages() + linear search with a direct
service.users().messages().get(format="full") call. Adds _extract_plain_text_body
helper for recursive MIME part walking. Update test to patch _get_gmail_service.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_scout_connectors_gmail.py

@@ -3,8 +3,7 @@
 from __future__ import annotations
 
 import uuid
-from datetime import datetime, timezone
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import MagicMock, patch
 
 import pytest
 
@@ -53,16 +52,24 @@ async def test_fetch_metadata_returns_subject_and_snippet():
 
 @pytest.mark.asyncio
 async def test_fetch_content_returns_body_text():
+    import base64
     scout = _make_scout()
     conn = GmailConnector()
-    # decrypt_token is patched because the test doesn't set OAUTH_ENCRYPTION_KEY.
-    with patch("app.scouts.connectors.gmail.decrypt_token", return_value={}), \
-         patch("app.scouts.connectors.gmail.GmailClient") as MockClient:
-        instance = MockClient.return_value
-        instance.fetch_messages = AsyncMock(return_value=[
-            MagicMock(id="msg-1", subject="S", sender="a@b", body_text="hello world",
-                     date=datetime.now(tz=timezone.utc), labels=[]),
-        ])
+    body_data = base64.urlsafe_b64encode(b"hello world").decode()
+    fake_message = {
+        "id": "msg-1",
+        "snippet": "hello world",
+        "payload": {
+            "mimeType": "text/plain",
+            "headers": [
+                {"name": "Subject", "value": "S"},
+                {"name": "From", "value": "a@b"},
+            ],
+            "body": {"data": body_data},
+        },
+    }
+    with patch("app.scouts.connectors.gmail._get_gmail_service") as mock_svc:
+        mock_svc.return_value.users().messages().get().execute.return_value = fake_message
         content = await conn.fetch_content(scout, ItemRef(source_msg_ref="msg-1"))
     assert content.body_text == "hello world"
     assert content.metadata.subject == "S"