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>

enterprise/app/services/captain/llm/translate_query_service.rb

@@ -27,9 +27,9 @@ class Captain::Llm::TranslateQueryService < Captain::BaseTaskService
   end
 
   # Translation is an internal operation, not customer-initiated.
-  # Prefer the system key; fall back to the account's hook key for self-hosted setups without one.
-  def api_key
-    @api_key ||= system_api_key.presence || openai_hook&.settings&.dig('api_key')
+  # It should always use the installation key.
+  def llm_credential
+    @llm_credential ||= system_llm_credential
   end
 
   def query_in_target_language?(query)

enterprise/lib/captain/conversation_completion_service.rb

@@ -56,12 +56,10 @@ class Captain::ConversationCompletionService < Captain::BaseTaskService
     { complete: false, reason: reason }
   end
 
-  # Prefer the system API key over the account's OpenAI hook key.
   # This is an internal operational evaluation, not a customer-triggered feature,
-  # so it should not consume the customer's OpenAI credits on hosted platforms.
-  # Falls back to the account hook for self-hosted deployments without a system key.
-  def api_key
-    @api_key ||= system_api_key.presence || openai_hook&.settings&.dig('api_key')
+  # so it should always use the installation key.
+  def llm_credential
+    @llm_credential ||= system_llm_credential
   end
 
   def event_name