feat[Go]: add feishu chat bot for chat channel (#17561)

### Summary

As title, related to #17520

internal/channels/feishu_test.go

@@ -0,0 +1,85 @@
+//
+//  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+package channels
+
+import (
+	"testing"
+	"time"
+
+	"ragflow/internal/channels/core"
+)
+
+func TestNewFeishuChannelFromConfigRequiresCredentials(t *testing.T) {
+	if _, err := newFeishuChannelFromConfig("account-1", map[string]any{"app_id": "app-1"}); err == nil {
+		t.Fatal("newFeishuChannelFromConfig succeeded without app_secret")
+	}
+}
+
+func TestNewFeishuChannelFromConfigNormalizesConfig(t *testing.T) {
+	ch, err := newFeishuChannelFromConfig("account-1", map[string]any{
+		"app_id":       "app-1",
+		"app_secret":   "secret-1",
+		"domain":       "lark",
+		"timeout_secs": "7",
+	})
+	if err != nil {
+		t.Fatalf("newFeishuChannelFromConfig returned error: %v", err)
+	}
+	if ch.account.AppID != "app-1" {
+		t.Fatalf("app id = %q, want app-1", ch.account.AppID)
+	}
+	if ch.account.AppSecret != "secret-1" {
+		t.Fatalf("app secret = %q, want secret-1", ch.account.AppSecret)
+	}
+	if ch.account.Domain != "lark" {
+		t.Fatalf("domain = %q, want lark", ch.account.Domain)
+	}
+	if ch.account.Timeout != 7*time.Second {
+		t.Fatalf("timeout = %s, want 7s", ch.account.Timeout)
+	}
+}
+
+func TestFeishuMessageText(t *testing.T) {
+	raw := `{"text":"hello"}`
+	if got := feishuMessageText(&raw); got != "hello" {
+		t.Fatalf("feishuMessageText() = %q, want hello", got)
+	}
+
+	plain := "hello"
+	if got := feishuMessageText(&plain); got != "hello" {
+		t.Fatalf("feishuMessageText() = %q, want raw fallback", got)
+	}
+}
+
+func TestFeishuEnqueueDoesNotMarkDroppedMessageSeen(t *testing.T) {
+	ch := newFeishuChannel(feishuAccount{AccountID: "account-1", AppID: "app-1", AppSecret: "secret-1"})
+	worker := &feishuWorker{queue: make(chan core.IncomingMessage, 1)}
+	worker.queue <- core.IncomingMessage{MessageID: "queued"}
+	ch.workers["chat-1"] = worker
+
+	ok := ch.enqueueIncoming(core.IncomingMessage{
+		ChatID:    "chat-1",
+		MessageID: "dropped",
+	})
+
+	if ok {
+		t.Fatal("enqueueIncoming succeeded for a full queue")
+	}
+	if _, seen := ch.seen["dropped"]; seen {
+		t.Fatal("dropped message was marked seen")
+	}
+}