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/thread_manager.rs

@@ -1,6 +1,7 @@
 use crate::AuthManager;
 use crate::CodexAuth;
 use crate::ModelProviderInfo;
+use crate::OPENAI_PROVIDER_ID;
 use crate::agent::AgentControl;
 use crate::codex::Codex;
 use crate::codex::CodexSpawnArgs;
@@ -168,6 +169,11 @@ impl ThreadManager {
         collaboration_modes_config: CollaborationModesConfig,
     ) -> Self {
         let codex_home = config.codex_home.clone();
+        let openai_models_provider = config
+            .model_providers
+            .get(OPENAI_PROVIDER_ID)
+            .cloned()
+            .unwrap_or_else(|| ModelProviderInfo::create_openai_provider(/* base_url */ None));
         let (thread_created_tx, _) = broadcast::channel(THREAD_CREATED_CHANNEL_CAPACITY);
         let plugins_manager = Arc::new(PluginsManager::new(codex_home.clone()));
         let mcp_manager = Arc::new(McpManager::new(Arc::clone(&plugins_manager)));
@@ -181,11 +187,12 @@ impl ThreadManager {
             state: Arc::new(ThreadManagerState {
                 threads: Arc::new(RwLock::new(HashMap::new())),
                 thread_created_tx,
-                models_manager: Arc::new(ModelsManager::new(
+                models_manager: Arc::new(ModelsManager::new_with_provider(
                     codex_home,
                     auth_manager.clone(),
                     config.model_catalog.clone(),
                     collaboration_modes_config,
+                    openai_models_provider,
                 )),
                 skills_manager,
                 plugins_manager,