[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/src/event_processor_with_jsonl_output.rs

@@ -430,6 +430,11 @@ impl EventProcessorWithJsonOutput {
                 }));
                 CodexStatus::Running
             }
+            ServerNotification::Warning(notification) => {
+                let warning = self.collect_warning(notification.message);
+                events.extend(warning.events);
+                warning.status
+            }
             ServerNotification::Error(notification) => {
                 let message = match notification.error.additional_details {
                     Some(details) if !details.is_empty() => {