refactor(channel): centralize retry around explicit send failures

Make channel delivery failures raise consistently so retry policy lives in ChannelManager rather than being split across individual channels. Tighten Telegram stream finalization, clarify sendMaxRetries semantics, and align the docs with the behavior the system actually guarantees.

tests/channels/test_telegram_channel.py

@@ -13,7 +13,7 @@ except ImportError:
 
 from nanobot.bus.events import OutboundMessage
 from nanobot.bus.queue import MessageBus
-from nanobot.channels.telegram import TELEGRAM_REPLY_CONTEXT_MAX_LEN, TelegramChannel
+from nanobot.channels.telegram import TELEGRAM_REPLY_CONTEXT_MAX_LEN, TelegramChannel, _StreamBuf
 from nanobot.channels.telegram import TelegramConfig
 
 
@@ -271,13 +271,30 @@ async def test_send_text_gives_up_after_max_retries() -> None:
     orig_delay = tg_mod._SEND_RETRY_BASE_DELAY
     tg_mod._SEND_RETRY_BASE_DELAY = 0.01
     try:
-        await channel._send_text(123, "hello", None, {})
+        with pytest.raises(TimedOut):
+            await channel._send_text(123, "hello", None, {})
     finally:
         tg_mod._SEND_RETRY_BASE_DELAY = orig_delay
 
     assert channel._app.bot.sent_messages == []
 
 
+@pytest.mark.asyncio
+async def test_send_delta_stream_end_raises_and_keeps_buffer_on_failure() -> None:
+    channel = TelegramChannel(
+        TelegramConfig(enabled=True, token="123:abc", allow_from=["*"]),
+        MessageBus(),
+    )
+    channel._app = _FakeApp(lambda: None)
+    channel._app.bot.edit_message_text = AsyncMock(side_effect=RuntimeError("boom"))
+    channel._stream_bufs["123"] = _StreamBuf(text="hello", message_id=7, last_edit=0.0)
+
+    with pytest.raises(RuntimeError, match="boom"):
+        await channel.send_delta("123", "", {"_stream_end": True})
+
+    assert "123" in channel._stream_bufs
+
+
 def test_derive_topic_session_key_uses_thread_id() -> None:
     message = SimpleNamespace(
         chat=SimpleNamespace(type="supergroup"),