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/suite/client.rs

@@ -715,7 +715,7 @@ async fn chatgpt_auth_sends_correct_request() {
     )
     .await;
 
-    let mut model_provider = built_in_model_providers()["openai"].clone();
+    let mut model_provider = built_in_model_providers(/* openai_base_url */ None)["openai"].clone();
     model_provider.base_url = Some(format!("{}/api/codex", server.uri()));
     let mut builder = test_codex()
         .with_auth(create_dummy_codex_auth())
@@ -791,7 +791,7 @@ async fn prefers_apikey_when_config_prefers_apikey_even_with_chatgpt_tokens() {
 
     let model_provider = ModelProviderInfo {
         base_url: Some(format!("{}/v1", server.uri())),
-        ..built_in_model_providers()["openai"].clone()
+        ..built_in_model_providers(/* openai_base_url */ None)["openai"].clone()
     };
 
     // Init session
@@ -1977,7 +1977,7 @@ async fn token_count_includes_rate_limits_snapshot() {
         .mount(&server)
         .await;
 
-    let mut provider = built_in_model_providers()["openai"].clone();
+    let mut provider = built_in_model_providers(/* openai_base_url */ None)["openai"].clone();
     provider.base_url = Some(format!("{}/v1", server.uri()));
 
     let mut builder = test_codex()