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.

nanobot/channels/manager.py

@@ -142,6 +142,14 @@ class ChannelManager:
             except asyncio.CancelledError:
                 break
 
+    @staticmethod
+    async def _send_once(channel: BaseChannel, msg: OutboundMessage) -> None:
+        """Send one outbound message without retry policy."""
+        if msg.metadata.get("_stream_delta") or msg.metadata.get("_stream_end"):
+            await channel.send_delta(msg.chat_id, msg.content, msg.metadata)
+        elif not msg.metadata.get("_streamed"):
+            await channel.send(msg)
+
     async def _send_with_retry(self, channel: BaseChannel, msg: OutboundMessage) -> None:
         """Send a message with retry on failure using exponential backoff.
 
@@ -151,12 +159,7 @@ class ChannelManager:
 
         for attempt in range(max_attempts):
             try:
-                if msg.metadata.get("_stream_delta") or msg.metadata.get("_stream_end"):
-                    await channel.send_delta(msg.chat_id, msg.content, msg.metadata)
-                elif msg.metadata.get("_streamed"):
-                    pass
-                else:
-                    await channel.send(msg)
+                await self._send_once(channel, msg)
                 return  # Send succeeded
             except asyncio.CancelledError:
                 raise  # Propagate cancellation for graceful shutdown