[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_human_output.rs

@@ -239,6 +239,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
                 );
                 CodexStatus::Running
             }
+            ServerNotification::Warning(notification) => self.process_warning(notification.message),
             ServerNotification::Error(notification) => {
                 eprintln!(
                     "{} {}",