fix(ai_loop): route send_result() errors to Discussion Hub as error entries (FR1, Bug #2)

Replaces deprecated ai_client.send() in _handle_request_event with
send_result() and branches on result.ok. On error, the first ErrorInfo
is routed to the event_queue as a 'response' with status='error',
allowing _on_comms_entry to add it to the discussion history.

The previous code called the @deprecated send() shim which silently
returns '' on error. The empty string was then filtered out by
_on_comms_entry (text_content.strip() check at line 3801), so users
saw no discussion entry for failed AI requests.

This also removes the dead 'except ai_client.ProviderError' clause at
line 3692 (the class was removed in commit 64b787b8). The 2 remaining
dead clauses at lines 305, 313 are fixed in the next commit (FR2).

src/app_controller.py

@@ -3673,28 +3673,26 @@ class AppController:
   ai_client.set_model_params(self.temperature, self.max_tokens, self.history_trunc_limit, self.top_p)
   ai_client.set_agent_tools(self.ui_agent_tools)  # Force update adapter path right before send to bypass potential duplication issues
   self._update_gcli_adapter(self.ui_gemini_cli_path)
-  try:
-   resp = ai_client.send(
-    event.stable_md, 
-    user_msg, 
-    event.base_dir, 
-    event.file_items, 
-    event.disc_text,
-    stream=True,
-    stream_callback=lambda text: self._on_ai_stream(text),
-    pre_tool_callback=self._confirm_and_run,
-    qa_callback=ai_client.run_tier4_analysis,
-    patch_callback=ai_client.run_tier4_patch_callback,
-    rag_engine=None # Already handled above
-   )
-   self.event_queue.put("response", {"text": resp, "status": "done", "role": "AI"})
+  result = ai_client.send_result(
+   event.stable_md,
+   user_msg,
+   event.base_dir,
+   event.file_items,
+   event.disc_text,
+   stream=True,
+   stream_callback=lambda text: self._on_ai_stream(text),
+   pre_tool_callback=self._confirm_and_run,
+   qa_callback=ai_client.run_tier4_analysis,
+   patch_callback=ai_client.run_tier4_patch_callback,
+   rag_engine=None, # Already handled above
+  )
+  if result.ok:
+   self.event_queue.put("response", {"text": result.data, "status": "done", "role": "AI"})
    self._ai_status = "done"
-  except ai_client.ProviderError as e:
-   self.event_queue.put("response", {"text": e.ui_message(), "status": "error", "role": "Vendor API"})
-   self._ai_status = f"error: {e.ui_message()}"
-  except Exception as e:
-   self.event_queue.put("response", {"text": f"ERROR: {e}", "status": "error", "role": "System"})
-   self._ai_status = f"error: {e}"
+  else:
+   err = result.errors[0]
+   self.event_queue.put("response", {"text": err.ui_message(), "status": "error", "role": "Vendor API"})
+   self._ai_status = f"error: {err.ui_message()}"
 
  def _on_tool_log(self, script: str, result: str) -> None:
   """