[codex] Surface runtime warnings in codex exec (#27415)

## Why

`codex exec` drops thread-scoped warning notifications. Warnings
discovered while a thread starts, including unreadable or invalid UTF-8
project `AGENTS.md` files, therefore become silent.

## What changed

- Process global and primary-thread warning notifications while
continuing to ignore warnings from unrelated threads.
- Render runtime warnings in human output and expose them through the
existing non-fatal error item in JSONL output.
- Add focused routing, rendering, and malformed project-instruction
coverage.

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

@@ -2,6 +2,8 @@
 
 use core_test_support::responses;
 use core_test_support::test_codex_exec::test_codex_exec;
+use predicates::prelude::PredicateBooleanExt;
+use predicates::str::contains;
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn exec_includes_workspace_agents_md_in_request() -> anyhow::Result<()> {
@@ -72,3 +74,71 @@ async fn exec_prefers_workspace_agents_override_md() -> anyhow::Result<()> {
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn exec_surfaces_project_instruction_loading_warnings() -> anyhow::Result<()> {
+    let test = test_codex_exec();
+    let project_agents_path = test.cwd_path().join("AGENTS.md");
+    std::fs::write(&project_agents_path, b"project\xFFinstructions")?;
+
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("resp1"),
+        responses::ev_assistant_message("m1", "fixture hello"),
+        responses::ev_completed("resp1"),
+    ]);
+    responses::mount_sse_once(&server, body).await;
+
+    test.cmd_with_server(&server)
+        .arg("--skip-git-repo-check")
+        .arg("tell me something")
+        .assert()
+        .success()
+        .stderr(contains("invalid UTF-8").and(contains(project_agents_path.display().to_string())));
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn exec_json_surfaces_project_instruction_loading_warnings() -> anyhow::Result<()> {
+    let test = test_codex_exec();
+    let project_agents_path = test.cwd_path().join("AGENTS.md");
+    std::fs::write(&project_agents_path, b"project\xFFinstructions")?;
+
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("resp1"),
+        responses::ev_assistant_message("m1", "fixture hello"),
+        responses::ev_completed("resp1"),
+    ]);
+    responses::mount_sse_once(&server, body).await;
+
+    let output = test
+        .cmd_with_server(&server)
+        .arg("--skip-git-repo-check")
+        .arg("--json")
+        .arg("tell me something")
+        .assert()
+        .success()
+        .get_output()
+        .stdout
+        .clone();
+    let events = String::from_utf8(output)?
+        .lines()
+        .map(serde_json::from_str::<serde_json::Value>)
+        .collect::<Result<Vec<_>, _>>()?;
+
+    assert!(
+        events.iter().any(|event| {
+            event["type"] == "item.completed"
+                && event["item"]["type"] == "error"
+                && event["item"]["message"].as_str().is_some_and(|message| {
+                    message.contains("invalid UTF-8")
+                        && message.contains(project_agents_path.display().to_string().as_str())
+                })
+        }),
+        "expected a JSONL warning event; observed: {events:?}"
+    );
+
+    Ok(())
+}