[6/6] Fail exec client operations after disconnect (#18027)

## Summary
- Reject new exec-server client operations once the transport has
disconnected.
- Convert pending RPC calls into closed errors instead of synthetic
server errors.
- Cover pending read and later write behavior after remote executor
disconnect.

## Verification
- `just fmt`
- `cargo check -p codex-exec-server`

## Stack
```text
@  #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
│
o  #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/exec-server/tests/exec_process.rs

@@ -4,6 +4,7 @@ mod common;
 
 use std::sync::Arc;
 
+use anyhow::Context;
 use anyhow::Result;
 use codex_exec_server::Environment;
 use codex_exec_server::ExecBackend;
@@ -484,6 +485,16 @@ async fn remote_exec_process_reports_transport_disconnect() -> Result<()> {
 
     let process = Arc::clone(&session.process);
     let mut events = process.subscribe_events();
+    let process_for_pending_read = Arc::clone(&process);
+    let pending_read = tokio::spawn(async move {
+        process_for_pending_read
+            .read(
+                /*after_seq*/ None,
+                /*max_bytes*/ None,
+                /*wait_ms*/ Some(60_000),
+            )
+            .await
+    });
     let server = context
         .server
         .as_mut()
@@ -499,6 +510,15 @@ async fn remote_exec_process_reports_transport_disconnect() -> Result<()> {
         "unexpected failure event: {event_message}"
     );
 
+    let pending_response = timeout(Duration::from_secs(2), pending_read).await???;
+    let pending_message = pending_response
+        .failure
+        .expect("pending read should surface disconnect as a failure");
+    assert!(
+        pending_message.starts_with("exec-server transport disconnected"),
+        "unexpected pending failure message: {pending_message}"
+    );
+
     let mut wake_rx = process.subscribe_wake();
     let response = read_process_until_change(process, &mut wake_rx, /*after_seq*/ None).await?;
     let message = response
@@ -513,6 +533,20 @@ async fn remote_exec_process_reports_transport_disconnect() -> Result<()> {
         "disconnect should close the process session"
     );
 
+    let write_result = timeout(
+        Duration::from_secs(2),
+        session.process.write(b"hello".to_vec()),
+    )
+    .await
+    .context("timed out waiting for write after disconnect")?;
+    let write_error = write_result.expect_err("write after disconnect should fail");
+    assert!(
+        write_error
+            .to_string()
+            .starts_with("exec-server transport disconnected"),
+        "unexpected write error: {write_error}"
+    );
+
     Ok(())
 }