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

tests/conftest.py

@@ -22,8 +22,10 @@ from openai.types.responses.response import IncompleteDetails
 import pytest
 
 from custom_components.openai_compatible.const import (
+    CONF_BASE_URL,
     CONF_CHAT_MODEL,
     DEFAULT_AI_TASK_NAME,
+    DEFAULT_BASE_URL,
     DEFAULT_CONVERSATION_NAME,
     DEFAULT_STT_NAME,
     DEFAULT_TTS_NAME,
@@ -65,6 +67,7 @@ def mock_config_entry(
         domain=DOMAIN,
         data={
             "api_key": "bla",
+            CONF_BASE_URL: DEFAULT_BASE_URL,
         },
         version=2,
         minor_version=7,