fix: log only on system api key failures (#13968)

Removes sentry flooding of unnecessary rubyllm logs of wrong API key.
Logs only system api key error since it would be P0.

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/llm/exception_trackable.rb

@@ -0,0 +1,11 @@
+module Llm::ExceptionTrackable
+  private
+
+  def capture_llm_exception(error, credential:)
+    if credential && credential[:source] == :system
+      ChatwootExceptionTracker.new(error, account: exception_tracking_account).capture_exception
+    else
+      Rails.logger.error("[LLM] account=#{exception_tracking_account&.id} #{error.class}: #{error.message}")
+    end
+  end
+end