feat: allow an empty API key for unauthenticated endpoints

The OpenAI SDK rejects api_key=None, so all three client sites now
fall back to a PLACEHOLDER_API_KEY ("not-required") when the entry has
no key, letting an unauthenticated LiteLLM/proxy endpoint be configured
with the API key field left blank.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01739Seac5eMAfNwFeiwV5r4

custom_components/openai_compatible/const.py

@@ -11,6 +11,7 @@ LOGGER: logging.Logger = logging.getLogger(__package__)
 
 CONF_BASE_URL = "base_url"
 DEFAULT_BASE_URL = "https://api.openai.com/v1"
+PLACEHOLDER_API_KEY = "not-required"
 
 DEFAULT_CONVERSATION_NAME = "OpenAI Conversation"
 DEFAULT_AI_TASK_NAME = "OpenAI AI Task"