fix(convertlib): keep an empty merge_forward body empty

ConvertBodyContent opened with `if ctx.RawContent == "" { return "" }` before the
converters moved down to internal/imcontent. The guard went with them, and
merge_forward is dispatched above that call — the shortcut-side converter expands
the tree from the API rather than from body.content, so it never reaches
imcontent's copy.

A merge_forward item whose body.content is an empty string therefore stopped
converting to "" the way every other message type does. With a prefetched page it
renders a full <forwarded_messages> subtree; without one, and with a runtime in
hand, it issues an inline GET /open-apis/im/v1/messages/{id} and prints
"[Merged forward: fetch failed: ...]" when that fails. Both reach the formatted
message output. FormatEventMessage carries no Runtime and no prefetch, so the
event path kept falling through to imcontent and was unaffected.

The guard belongs above the dispatch, where it was, and now also covers a nil
context, which the original would have dereferenced. Two tests pin it: the
prefetch path must still convert to "", and the runtime path must issue zero
requests.

shortcuts/im/convert_lib/content_convert.go

@@ -41,7 +41,17 @@ type ConvertContext struct {
 }
 
 // ConvertBodyContent converts body.content (a raw JSON string) to human-readable text.
+//
+// The empty-content guard has to live here, above the merge_forward dispatch,
+// not only in imcontent.ConvertBodyContent: the shortcut-side merge_forward
+// converter never consults imcontent on its expansion paths, so a guard that
+// only sat below the dispatch would let an empty-content merge_forward render a
+// prefetched <forwarded_messages> tree — or issue an inline
+// GET /open-apis/im/v1/messages/{id} — where every other message type returns "".
 func ConvertBodyContent(msgType string, ctx *ConvertContext) string {
+	if ctx == nil || ctx.RawContent == "" {
+		return ""
+	}
 	if msgType == "merge_forward" {
 		return (mergeForwardConverter{}).Convert(ctx)
 	}

shortcuts/im/convert_lib/merge_test.go

@@ -357,3 +357,65 @@ func TestFormatMergeForwardSubTreeInteractiveCardUsesMentions(t *testing.T) {
 		t.Fatalf("FormatMergeForwardSubTree(interactive card) = %s", got)
 	}
 }
+
+// TestConvertBodyContentEmptyMergeForwardStaysEmpty pins the empty-content
+// guard that sits above the merge_forward dispatch. A merge_forward item whose
+// body.content is an empty string (recalled/edited containers come back this
+// way) must convert to "" — the same as every other message type — and must
+// not reach the converter at all: reaching it would render a
+// <forwarded_messages> tree from a prefetched cache, or issue a
+// GET /open-apis/im/v1/messages/{id} when no prefetch is present.
+func TestConvertBodyContentEmptyMergeForwardStaysEmpty(t *testing.T) {
+	prefetch := map[string][]map[string]interface{}{
+		"om_root": {
+			{
+				"message_id":  "om_child",
+				"msg_type":    "text",
+				"create_time": "1710500000000",
+				"sender":      map[string]interface{}{"id": "ou_alice", "name": "Alice"},
+				"body":        map[string]interface{}{"content": `{"text":"hello"}`},
+			},
+		},
+	}
+
+	if got := ConvertBodyContent("merge_forward", &ConvertContext{
+		MessageID:            "om_root",
+		MergeForwardSubItems: prefetch,
+	}); got != "" {
+		t.Fatalf("ConvertBodyContent(merge_forward, empty content, prefetch hit) = %q, want empty", got)
+	}
+
+	item := map[string]interface{}{
+		"message_id":  "om_root",
+		"msg_type":    "merge_forward",
+		"create_time": "1710500000000",
+		"body":        map[string]interface{}{"content": ""},
+	}
+	msg := FormatMessageItemWithMergePrefetch(item, nil, nil, prefetch)
+	if got, _ := msg["content"].(string); got != "" {
+		t.Fatalf("FormatMessageItem(merge_forward, empty content, prefetch hit) content = %q, want empty", got)
+	}
+}
+
+// TestConvertBodyContentEmptyMergeForwardIssuesNoRequest is the network half of
+// the guard: with no prefetch and a live runtime, an empty-content
+// merge_forward must not fall into the inline-fetch slow path.
+func TestConvertBodyContentEmptyMergeForwardIssuesNoRequest(t *testing.T) {
+	var requests int
+	runtime := newBotConvertlibRuntime(t, convertlibRoundTripFunc(func(req *http.Request) (*http.Response, error) {
+		requests++
+		return nil, fmt.Errorf("unexpected request: %s", req.URL.String())
+	}))
+
+	got := ConvertBodyContent("merge_forward", &ConvertContext{
+		MessageID:   "om_root",
+		Runtime:     runtime,
+		SenderNames: map[string]string{},
+	})
+	if got != "" {
+		t.Fatalf("ConvertBodyContent(merge_forward, empty content, runtime set) = %q, want empty", got)
+	}
+	if requests != 0 {
+		t.Fatalf("ConvertBodyContent(merge_forward, empty content) issued %d HTTP request(s), want 0", requests)
+	}
+}