telegram: lower MaxMessageLength to 4000 for HTML expansion margin

The Manager splits at MaxMessageLength before calling Send(), and
Telegram's Send() was re-splitting at 4000 internally. Aligning the
channel-level limit to 4000 avoids that redundant second split while
preserving the safety margin for markdown-to-HTML expansion.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/channels/telegram/telegram.go

@@ -86,7 +86,7 @@ func NewTelegramChannel(cfg *config.Config, bus *bus.MessageBus) (*TelegramChann
 		telegramCfg,
 		bus,
 		telegramCfg.AllowFrom,
-		channels.WithMaxMessageLength(4096),
+		channels.WithMaxMessageLength(4000),
 		channels.WithGroupTrigger(telegramCfg.GroupTrigger),
 		channels.WithReasoningChannelID(telegramCfg.ReasoningChannelID),
 	)

pkg/channels/telegram/telegram_test.go

@@ -69,7 +69,7 @@ func newTestChannel(t *testing.T, caller *stubCaller) *TelegramChannel {
 	require.NoError(t, err)
 
 	base := channels.NewBaseChannel("telegram", nil, nil, nil,
-		channels.WithMaxMessageLength(4096),
+		channels.WithMaxMessageLength(4000),
 	)
 	base.SetRunning(true)