refactor(yuanbao): migrate recall to load_transcript()

Yuanbao's recall feature was reading the gateway JSONL directly to look up
messages by platform message_id, which state.db does not preserve. Migrated
to use load_transcript() which returns DB messages.

Recall branch A1 (message_id match) now falls through to A2 (content match)
or B (system note) for all sessions — a documented degradation. Follow-up
issue: add platform_message_id column to state.db messages to restore
exact-id matching.

tests/gateway/platforms/test_yuanbao_recall_db_only.py

@@ -0,0 +1,25 @@
+"""Yuanbao recall: branch A2 (content-match) works without JSONL message_id."""
+from gateway.session import SessionStore
+from gateway.config import GatewayConfig
+
+
+def test_recall_falls_through_to_content_match_without_message_id(tmp_path):
+    """When transcript has no message_id field, A2 content-match still works."""
+    config = GatewayConfig()
+    store = SessionStore(sessions_dir=tmp_path, config=config)
+
+    sid = "test-yuanbao-recall"
+    store._db.create_session(session_id=sid, source="yuanbao:group:G")
+    store.append_to_transcript(sid, {"role": "user", "content": "sensitive content", "timestamp": 1.0})
+    store.append_to_transcript(sid, {"role": "assistant", "content": "ack", "timestamp": 2.0})
+
+    # The post-PR state: load_transcript returns DB-only, no message_id field.
+    history = store.load_transcript(sid)
+    assert all("message_id" not in msg for msg in history), \
+        "DB-only history should not carry message_id"
+
+    # Branch A2: content match should still find the message
+    target = next((m for m in history
+                   if m.get("role") == "user" and m.get("content") == "sensitive content"), None)
+    assert target is not None
+    # Caller would then redact: target["content"] = REDACTED; store.rewrite_transcript(sid, history)