Only drop the LLM API key when it was not submitted

Keep the recommended value as the fallback for a subentry with no
llm_hass_api key, and only remove the key when it is absent from the
submitted input, so an empty selection is stored and read back as empty.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016D21QGmGduj7t1aQLX26pS

homeassistant/components/open_router/config_flow.py

@@ -146,6 +146,8 @@ class ConversationFlowHandler(OpenRouterSubentryFlowHandler):
             return self.async_abort(reason="entry_not_loaded")
 
         if user_input is not None:
+            if user_input.get(CONF_LLM_HASS_API) is None:
+                user_input.pop(CONF_LLM_HASS_API, None)
             if self._is_new:
                 return self.async_create_entry(
                     title=self.models[user_input[CONF_MODEL]].name, data=user_input