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/tests/common/test_codex.rs

@@ -226,7 +226,7 @@ impl TestCodexBuilder {
     ) -> anyhow::Result<(Config, Arc<TempDir>)> {
         let model_provider = ModelProviderInfo {
             base_url: Some(base_url),
-            ..built_in_model_providers()["openai"].clone()
+            ..built_in_model_providers(/* openai_base_url */ None)["openai"].clone()
         };
         let cwd = Arc::new(TempDir::new()?);
         let mut config = load_default_config_for_test(home).await;