[3/6] Add pushed exec process events (#18020)

## Summary
- Add a pushed `ExecProcessEvent` stream alongside retained
`process/read` output.
- Publish local and remote output, exit, close, and failure events.
- Cover the event stream with shared local/remote exec process tests.

## Testing
- `cargo check -p codex-exec-server`
- `cargo check -p codex-rmcp-client`
- Not run: `cargo test` per repo instruction; CI will cover.

## Stack
```text
o  #18027 [6/6] Fail exec client operations after disconnect
│
o  #18212 [5/6] Wire executor-backed MCP stdio
│
o  #18087 [4/6] Abstract MCP stdio server launching
│
@  #18020 [3/6] Add pushed exec process events
│
o  #18086 [2/6] Support piped stdin in exec process API
│
o  #18085 [1/6] Add MCP server environment config
│
o  main
```

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/unified_exec/process_tests.rs

@@ -2,6 +2,7 @@ use super::process::UnifiedExecProcess;
 use crate::unified_exec::UnifiedExecError;
 use async_trait::async_trait;
 use codex_exec_server::ExecProcess;
+use codex_exec_server::ExecProcessEventReceiver;
 use codex_exec_server::ExecServerError;
 use codex_exec_server::ProcessId;
 use codex_exec_server::ReadResponse;
@@ -33,6 +34,10 @@ impl ExecProcess for MockExecProcess {
         self.wake_tx.subscribe()
     }
 
+    fn subscribe_events(&self) -> ExecProcessEventReceiver {
+        ExecProcessEventReceiver::empty()
+    }
+
     async fn read(
         &self,
         _after_seq: Option<u64>,