feat(alias): log the shape of a capture at debug level

The capture failures worth debugging are all about what Telegram did not
deliver: a reply stripped of its content, a caption where text was
expected, or a kind that falls through the switch. None of that is
visible from the user-facing refusal, which only says "unsupported".

One alias_capture line per /alias reports field names, lengths and
counts — never message text, so aliased messages cannot travel with the
logs.

internal/modules/alias/alias_debug_test.go

@@ -0,0 +1,106 @@
+package alias
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/go-telegram/bot/models"
+)
+
+// attrs renders replyShape's key/value pairs into one string for assertions.
+func attrs(t *testing.T, replied *models.Message) string {
+	t.Helper()
+	entry, ok := capture(replied)
+	pairs := replyShape(replied, entry, ok)
+	if len(pairs)%2 != 0 {
+		t.Fatalf("replyShape returned %d values, want key/value pairs", len(pairs))
+	}
+	var sb strings.Builder
+	for i := 0; i < len(pairs); i += 2 {
+		fmt.Fprintf(&sb, "%v=%v ", pairs[i], pairs[i+1])
+	}
+	return sb.String()
+}
+
+func TestReplyShape(t *testing.T) {
+	cases := []struct {
+		name    string
+		replied *models.Message
+		want    []string
+	}{
+		{
+			// The case this exists for: /alias arrives with no reply attached,
+			// which looks the same to the caller as forgetting to reply.
+			name:    "absent reply",
+			replied: nil,
+			want:    []string{"reply=absent"},
+		},
+		{
+			// What another bot's message looks like once Telegram strips it:
+			// delivered, from a bot, carrying nothing.
+			name:    "stripped bot reply",
+			replied: &models.Message{ID: 9, From: &models.User{ID: 555, IsBot: true}},
+			want:    []string{"reply=present", "fields=none", "captured=false", "from_bot=true", "from_id=555"},
+		},
+		{
+			name: "text reply",
+			replied: &models.Message{
+				ID:       4,
+				From:     &models.User{ID: 7},
+				Text:     "hello",
+				Entities: []models.MessageEntity{{Type: models.MessageEntityTypeBold, Length: 5}},
+			},
+			want: []string{"fields=text", "text_len=5", "entities=1", "captured=true", "kind=text", "from_bot=false"},
+		},
+		{
+			// A kind capture refuses still reports what arrived, so
+			// "unsupported" can be told apart from "empty".
+			name:    "unsupported kind",
+			replied: &models.Message{ID: 5, From: &models.User{ID: 7}, Location: &models.Location{Latitude: 1}},
+			want:    []string{"fields=location", "captured=false"},
+		},
+		{
+			name: "photo with caption",
+			replied: &models.Message{
+				ID:      6,
+				From:    &models.User{ID: 7},
+				Photo:   []models.PhotoSize{{FileID: "p"}},
+				Caption: "cap",
+			},
+			want: []string{"caption", "photo", "caption_len=3", "kind=photo"},
+		},
+		{
+			// No From at all — an anonymous channel post, or a stripped reply.
+			name:    "no sender",
+			replied: &models.Message{ID: 7, SenderChat: &models.Chat{ID: -100}},
+			want:    []string{"from=absent", "sender_chat=-100"},
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := attrs(t, tc.replied)
+			for _, want := range tc.want {
+				if !strings.Contains(got, want) {
+					t.Errorf("replyShape = %q, missing %q", got, want)
+				}
+			}
+		})
+	}
+}
+
+// The debug line reports shape, never content: it lands in stdout and whatever
+// ships it, so aliased message text must not travel with it.
+func TestReplyShape_LogsNoMessageContent(t *testing.T) {
+	const secret = "SENSITIVE-MESSAGE-BODY"
+	replied := &models.Message{
+		ID:      1,
+		From:    &models.User{ID: 7, FirstName: secret, Username: secret},
+		Text:    secret,
+		Caption: secret,
+	}
+
+	if got := attrs(t, replied); strings.Contains(got, secret) {
+		t.Errorf("replyShape leaked message content: %q", got)
+	}
+}