Support CODEX_API_KEY for codex exec (#4615)

Allows to set API key per invocation of `codex exec`

codex-rs/exec/tests/suite/auth_env.rs

@@ -0,0 +1,34 @@
+#![allow(clippy::unwrap_used, clippy::expect_used)]
+use core_test_support::responses::ev_completed;
+use core_test_support::responses::sse;
+use core_test_support::responses::sse_response;
+use core_test_support::responses::start_mock_server;
+use core_test_support::test_codex_exec::test_codex_exec;
+use wiremock::Mock;
+use wiremock::matchers::header;
+use wiremock::matchers::method;
+use wiremock::matchers::path;
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn exec_uses_codex_api_key_env_var() -> anyhow::Result<()> {
+    let test = test_codex_exec();
+    let server = start_mock_server().await;
+
+    Mock::given(method("POST"))
+        .and(path("/v1/responses"))
+        .and(header("Authorization", "Bearer dummy"))
+        .respond_with(sse_response(sse(vec![ev_completed("request_0")])))
+        .expect(1)
+        .mount(&server)
+        .await;
+
+    test.cmd_with_server(&server)
+        .arg("--skip-git-repo-check")
+        .arg("-C")
+        .arg(env!("CARGO_MANIFEST_DIR"))
+        .arg("echo testing codex api key")
+        .assert()
+        .success();
+
+    Ok(())
+}