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;

codex-rs/core/tests/common/test_codex_exec.rs

@@ -23,7 +23,8 @@ impl TestCodexExecBuilder {
     pub fn cmd_with_server(&self, server: &MockServer) -> assert_cmd::Command {
         let mut cmd = self.cmd();
         let base = format!("{}/v1", server.uri());
-        cmd.env("OPENAI_BASE_URL", base);
+        cmd.arg("-c")
+            .arg(format!("openai_base_url={}", toml_string_literal(&base)));
         cmd
     }
 
@@ -35,6 +36,10 @@ impl TestCodexExecBuilder {
     }
 }
 
+fn toml_string_literal(value: &str) -> String {
+    serde_json::to_string(value).expect("serialize TOML string literal")
+}
+
 pub fn test_codex_exec() -> TestCodexExecBuilder {
     TestCodexExecBuilder {
         home: TempDir::new().expect("create temp home"),