Don't 500 on channel sends with an unresolvable @mention

A channel post containing an @token that is not a syntactically valid
username (too short, leading digit, etc.) made messages.sendMessage return
500 INTERNAL_SERVER_ERROR: mentionedUserIDsFromMessage turned every
ResolveUsername error into internalErr().

Skip tokens that fail with ErrUsernameInvalid / ErrUsernameNotOccupied
instead, matching real Telegram (the message sends, the client renders the
mention and only fails to open it on tap). Only unexpected storage errors
still abort. Same fix applied to the bot send path.

internal/rpc/channels_updates_rpc_test.go

@@ -640,6 +640,39 @@ func TestChannelUnreadMentionsRPCUsesMentionState(t *testing.T) {
 	}
 }
 
+// A channel post containing an @token that is not a syntactically valid
+// username (too short, leading digit, etc.) must still send. Real Telegram
+// renders it as a mention and only fails when the reader taps it; it never
+// rejects the send. Regression for a 500 INTERNAL_SERVER_ERROR.
+func TestChannelSendMessageWithUnresolvableMentionSucceeds(t *testing.T) {
+	ctx := context.Background()
+	userStore := memory.NewUserStore()
+	owner, _ := userStore.Create(ctx, domain.User{AccessHash: 9201, Phone: "15550009201", FirstName: "Owner", Username: "owner_badmention"})
+	channelStore := memory.NewChannelStore()
+	channelService := appchannels.NewService(channelStore)
+	r := New(Config{}, Deps{
+		Users:    appusers.NewService(userStore),
+		Channels: channelService,
+	}, zaptest.NewLogger(t), clock.System)
+	created, err := channelService.CreateMegagroupFromCreateChat(ctx, owner.ID, domain.CreateChannelRequest{
+		Title: "Bad Mention",
+		Date:  1700009201,
+	})
+	if err != nil {
+		t.Fatalf("create megagroup: %v", err)
+	}
+	peer := &tg.InputPeerChannel{ChannelID: created.Channel.ID, AccessHash: created.Channel.AccessHash}
+	for i, text := range []string{"look at @zio", "hi @2cool and @_x"} {
+		if _, err := r.onMessagesSendMessage(WithUserID(ctx, owner.ID), &tg.MessagesSendMessageRequest{
+			Peer:     peer,
+			Message:  text,
+			RandomID: int64(9202001 + i),
+		}); err != nil {
+			t.Fatalf("send %q: unexpected error %v", text, err)
+		}
+	}
+}
+
 func TestChannelDifferenceIncludesExtraForwardSourceChannel(t *testing.T) {
 	channel := domain.Channel{ID: 2000000100, AccessHash: 9010, Title: "Megagroup", Megagroup: true, Date: 1700000000, Pts: 3}
 	source := domain.Channel{ID: 2000000101, AccessHash: 9011, Title: "Source", Broadcast: true, Date: 1700000000}

internal/rpc/messages_bot_no_state.go

@@ -341,7 +341,10 @@ func (r *Router) mentionedUserIDsFromDomainMessage(ctx context.Context, currentU
 		for _, username := range extractMentionUsernames(message, domain.MaxChannelMentionRecipients-len(out)) {
 			user, found, err := identity.ResolveUsername(ctx, currentUserID, username)
 			if err != nil {
-				return nil, internalErr()
+				if mentionResolveFatal(err) {
+					return nil, internalErr()
+				}
+				continue
 			}
 			if found {
 				add(user.ID)

internal/rpc/messages_send.go

@@ -437,7 +437,10 @@ func (r *Router) mentionedUserIDsFromMessage(ctx context.Context, currentUserID
 		for _, username := range extractMentionUsernames(message, domain.MaxChannelMentionRecipients-len(out)) {
 			user, found, err := identity.ResolveUsername(ctx, currentUserID, username)
 			if err != nil {
-				return nil, internalErr()
+				if mentionResolveFatal(err) {
+					return nil, internalErr()
+				}
+				continue
 			}
 			if found {
 				add(user.ID)
@@ -450,6 +453,17 @@ func (r *Router) mentionedUserIDsFromMessage(ctx context.Context, currentUserID
 	return out, nil
 }
 
+// mentionResolveFatal reports whether a ResolveUsername error while scanning
+// message text for @mentions should abort the send. A syntactically invalid or
+// unoccupied @token is not a failure: real Telegram sends the message, renders
+// the token as a mention, and only fails to open it when the reader taps it.
+// Only an unexpected (storage) error aborts the RPC.
+func mentionResolveFatal(err error) bool {
+	return err != nil &&
+		!errors.Is(err, domain.ErrUsernameInvalid) &&
+		!errors.Is(err, domain.ErrUsernameNotOccupied)
+}
+
 func extractMentionUsernames(message string, limit int) []string {
 	if limit <= 0 || message == "" {
 		return nil