feat(gateway): 消息mode落库(A3) — 持久化真实写入渲染模式

- POST /api/v1/chat 与 ws.chat 上行新增可选 mode（行为模式 design/01 §3/design/16 §5.1）
- Web路径经ctx透传本轮mode到编排器持久化(persistUserMessages/persistAssistantReply)，缺省回落chat
- pendingMsg记录mode，恢复/续接路径沿用发起轮次mode
- IM路径(persistToMain/persistIsolated)恒为chat
- 新增/扩充测试：REST/WS/orchestrator/channelbridge 覆盖mode写入
- Docs/api-client.md v0.20 同步mode字段

internal/services/channelbridge/channelbridge_test.go

@@ -315,14 +315,17 @@ func TestOwnerPrivatePersistsToMainSessionWithPlatform(t *testing.T) {
 	if len(recs) != 2 {
 		t.Fatalf("appends = %d, want 2 (user + assistant)", len(recs))
 	}
-	// Both land on the main web session and carry the channel platform + adapter.
+	// Both land on the main web session and carry the channel platform + adapter,
+	// and IM messages are always persisted in chat mode (design/16 §5.1).
 	if recs[0].SessionID != history.MainSessionID || recs[0].Role != history.RoleUser ||
-		recs[0].Content != "hello" || recs[0].Platform != "telegram" || recs[0].AdapterID != "telegram" {
-		t.Fatalf("user record = %+v, want web/telegram/hello/telegram", recs[0])
+		recs[0].Content != "hello" || recs[0].Platform != "telegram" || recs[0].AdapterID != "telegram" ||
+		recs[0].Mode != "chat" {
+		t.Fatalf("user record = %+v, want web/telegram/hello/telegram/chat", recs[0])
 	}
 	if recs[1].SessionID != history.MainSessionID || recs[1].Role != history.RoleAssistant ||
-		recs[1].Content != "微信回复" || recs[1].Platform != "telegram" || recs[1].AdapterID != "telegram" {
-		t.Fatalf("assistant record = %+v, want web/telegram/微信回复/telegram", recs[1])
+		recs[1].Content != "微信回复" || recs[1].Platform != "telegram" || recs[1].AdapterID != "telegram" ||
+		recs[1].Mode != "chat" {
+		t.Fatalf("assistant record = %+v, want web/telegram/微信回复/telegram/chat", recs[1])
 	}
 	// Every persisted message was handed to the broadcast hook.
 	if len(persisted) != 2 {
@@ -414,6 +417,10 @@ func TestGroupMessageNotPersistedToMainSession(t *testing.T) {
 		if rec.Platform != "telegram" {
 			t.Fatalf("record[%d] Platform = %q, want telegram", i, rec.Platform)
 		}
+		// Isolated IM messages are still chat-mode (design/16 §5.1).
+		if rec.Mode != "chat" {
+			t.Fatalf("record[%d] Mode = %q, want chat", i, rec.Mode)
+		}
 	}
 	if chat.sessionID != "sess-g" {
 		t.Fatalf("group loop session id = %q, want sess-g", chat.sessionID)