chore(MAT): log stream summary so deploys can be verified at a glance

Single info line per _stream_chat_completion call reporting model,
character count, tool-call count, and whether a Matrix message was
progressively streamed. Lets us confirm the streaming path is active
without tracing sentry breadcrumbs.

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

bot.py

@@ -3815,6 +3815,10 @@ class Bot:
         tc_list = None
         if tool_calls_acc:
             tc_list = [tool_calls_acc[i] for i in sorted(tool_calls_acc.keys())]
+        logger.info(
+            "[stream] model=%s chars=%d tool_calls=%d streamed_to_matrix=%s",
+            model, len(content), len(tc_list or []), event_id is not None,
+        )
         return content, tc_list, usage, event_id
 
     async def _get_call_encryption_key(self, room_id: str, sender: str, caller_device_id: str = "") -> bytes | None: