Add openai_base_url config override for built-in provider (#12031)

We regularly get bug reports from users who mistakenly have the
`OPENAI_BASE_URL` environment variable set. This PR deprecates this
environment variable in favor of a top-level config key
`openai_base_url` that is used for the same purpose. By making it a
config key, it will be more visible to users. It will also participate
in all of the infrastructure we've added for layered and managed
configs.

Summary
- introduce the `openai_base_url` top-level config key, update
schema/tests, and route the built-in openai provider through it while
- fall back to deprecated `OPENAI_BASE_URL` env var but warn user of
deprecation when no `openai_base_url` config key is present
- update CLI, SDK, and TUI code to prefer the new config path (with a
deprecated env-var fallback) and document the SDK behavior change

codex-rs/core/src/config/config_tests.rs

@@ -4134,7 +4134,7 @@ model_verbosity = "high"
         supports_websockets: false,
     };
     let model_provider_map = {
-        let mut model_provider_map = built_in_model_providers();
+        let mut model_provider_map = built_in_model_providers(/* openai_base_url */ None);
         model_provider_map.insert("openai-custom".to_string(), openai_custom_provider.clone());
         model_provider_map
     };