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/strings.json

@@ -21,10 +21,12 @@
       },
       "user": {
         "data": {
-          "api_key": "[%key:common::config_flow::data::api_key%]"
+          "api_key": "[%key:common::config_flow::data::api_key%]",
+          "base_url": "API base URL"
         },
         "data_description": {
-          "api_key": "Your OpenAI API key."
+          "api_key": "Your OpenAI API key.",
+          "base_url": "The OpenAI-compatible endpoint, e.g. http://192.0.2.10:4000/v1 for LiteLLM. Must support the Responses API."
         },
         "description": "Set up OpenAI Conversation integration by providing your OpenAI API key. Instructions to obtain an API key can be found [here]({instructions_url})."
       }