feat: configurable base_url threaded to all three client sites

Adds CONF_BASE_URL/DEFAULT_BASE_URL and threads base_url through every
AsyncOpenAI construction site: config_flow.validate_input,
__init__.async_setup_entry, and config_flow's web-search geolocation
helper (_get_location_data) — the easy one to miss, since it would
otherwise silently talk to api.openai.com when the user configured a
different endpoint.

Fixes upstream-ported fixtures (conftest.mock_config_entry,
test_config_flow's inline MockConfigEntry/data-schema assertions) to
supply base_url now that the config-flow schema requires it.

No translations/en.json exists in this repo (only strings.json), so
only strings.json got the new "base_url" UI strings.

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

custom_components/openai_compatible/const.py

@@ -9,6 +9,9 @@ from homeassistant.helpers import llm
 DOMAIN = "openai_compatible"
 LOGGER: logging.Logger = logging.getLogger(__package__)
 
+CONF_BASE_URL = "base_url"
+DEFAULT_BASE_URL = "https://api.openai.com/v1"
+
 DEFAULT_CONVERSATION_NAME = "OpenAI Conversation"
 DEFAULT_AI_TASK_NAME = "OpenAI AI Task"
 DEFAULT_STT_NAME = "OpenAI STT"