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/__init__.py

@@ -39,6 +39,7 @@ from homeassistant.helpers.httpx_client import get_async_client
 from homeassistant.helpers.typing import UNDEFINED, ConfigType, UndefinedType
 
 from .const import (
+    CONF_BASE_URL,
     CONF_CHAT_MODEL,
     CONF_FILENAMES,
     CONF_MAX_TOKENS,
@@ -282,6 +283,7 @@ async def async_setup_entry(hass: HomeAssistant, entry: OpenAIConfigEntry) -> bo
     """Set up OpenAI Conversation from a config entry."""
     client = openai.AsyncOpenAI(
         api_key=entry.data[CONF_API_KEY],
+        base_url=entry.data[CONF_BASE_URL],
         http_client=get_async_client(hass),
     )