[codex] Handle Ctrl-C for non-TTY unified exec (#26734)

## Why

A long-running unified exec process started with `tty: false` could not
be interrupted via `write_stdin`: ordinary non-TTY stdin writes are
rejected once stdin is closed, but an exact U+0003 payload should still
map to a process interrupt. The interrupt should flow through the same
process lifecycle path as a real signal so Codex preserves
process-reported output and exit metadata instead of fabricating a
Ctrl-C exit code or tearing down the session early.

## What Changed

- Add `process/signal` to exec-server with `ProcessSignal::Interrupt`
and an empty response.
- Add a non-consuming `ProcessHandle::signal` path for spawned
processes; on Unix it sends SIGINT to the process group and leaves
terminate/hard-kill unchanged.
- Route non-TTY U+0003 `write_stdin` through `process.signal(...)`
instead of `terminate`, then let the normal post-write collection path
drain output and observe exit.
- Add exec-server coverage where a shell `trap INT` handler prints the
signal and exits with its own code.
- Add unified exec coverage where a `tty: false` process traps SIGINT,
emits output, and exits with its own code.

## Validation

- `just test -p codex-exec-server
exec_process_signal_interrupts_process`
- `just test -p codex-exec-server`
- `just test -p codex-core
write_stdin_ctrl_c_interrupts_non_tty_session`

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

@@ -14,6 +14,7 @@ use tokio_util::sync::CancellationToken;
 
 use crate::exec::is_likely_sandbox_denied;
 use codex_exec_server::ExecProcess;
+use codex_exec_server::ProcessSignal as ExecServerProcessSignal;
 use codex_exec_server::ReadResponse as ExecReadResponse;
 use codex_exec_server::StartedExecProcess;
 use codex_exec_server::WriteStatus;
@@ -23,6 +24,7 @@ use codex_protocol::protocol::TruncationPolicy;
 use codex_sandboxing::SandboxType;
 use codex_utils_output_truncation::formatted_truncate_text;
 use codex_utils_pty::ExecCommandSession;
+use codex_utils_pty::ProcessSignal as PtyProcessSignal;
 use codex_utils_pty::SpawnedPty;
 
 use super::UNIFIED_EXEC_OUTPUT_MAX_TOKENS;
@@ -31,7 +33,6 @@ use super::head_tail_buffer::HeadTailBuffer;
 use super::process_state::ProcessState;
 
 const EARLY_EXIT_GRACE_PERIOD: Duration = Duration::from_millis(150);
-
 pub(crate) trait SpawnLifecycle: std::fmt::Debug + Send + Sync {
     /// Returns file descriptors that must stay open across the child `exec()`.
     ///
@@ -212,6 +213,18 @@ impl UnifiedExecProcess {
         }
     }
 
+    pub(super) async fn interrupt(&self) -> Result<(), UnifiedExecError> {
+        match &self.process_handle {
+            ProcessHandle::Local(process_handle) => process_handle
+                .signal(PtyProcessSignal::Interrupt)
+                .map_err(|err| UnifiedExecError::process_failed(err.to_string())),
+            ProcessHandle::ExecServer(process_handle) => process_handle
+                .signal(ExecServerProcessSignal::Interrupt)
+                .await
+                .map_err(|err| UnifiedExecError::process_failed(err.to_string())),
+        }
+    }
+
     pub(super) fn fail_and_terminate(&self, message: String) {
         let state = self.state_rx.borrow().clone();
         if state.failure_message.is_none() {

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

@@ -72,6 +72,7 @@ const UNIFIED_EXEC_ENV: [(&str, &str); 10] = [
 const NETWORK_ACCESS_DENIED_MESSAGE: &str =
     "Network access was denied by the Codex sandbox network proxy.";
 const LATE_NETWORK_DENIAL_GRACE_PERIOD: Duration = Duration::from_millis(100);
+const INTERRUPT: &str = "\u{3}";
 
 /// Test-only override for deterministic unified exec process IDs.
 ///
@@ -617,24 +618,29 @@ impl UnifiedExecProcessManager {
 
         if !request.input.is_empty() {
             if !tty {
-                return Err(UnifiedExecError::StdinClosed);
-            }
-            match process.write(request.input.as_bytes()).await {
-                Ok(()) => {
-                    // Give the remote process a brief window to react so that we are
-                    // more likely to capture its output in the poll below.
-                    tokio::time::sleep(Duration::from_millis(100)).await;
+                if request.input == INTERRUPT {
+                    process.interrupt().await?;
+                } else {
+                    return Err(UnifiedExecError::StdinClosed);
                 }
-                Err(err) => {
-                    let status = self.refresh_process_state(process_id).await;
-                    if matches!(status, ProcessStatus::Exited { .. }) {
-                        status_after_write = Some(status);
-                    } else if matches!(err, UnifiedExecError::ProcessFailed { .. }) {
-                        process.terminate();
-                        self.release_process_id(process_id).await;
-                        return Err(err);
-                    } else {
-                        return Err(err);
+            } else {
+                match process.write(request.input.as_bytes()).await {
+                    Ok(()) => {
+                        // Give the remote process a brief window to react so that we are
+                        // more likely to capture its output in the poll below.
+                        tokio::time::sleep(Duration::from_millis(100)).await;
+                    }
+                    Err(err) => {
+                        let status = self.refresh_process_state(process_id).await;
+                        if matches!(status, ProcessStatus::Exited { .. }) {
+                            status_after_write = Some(status);
+                        } else if matches!(err, UnifiedExecError::ProcessFailed { .. }) {
+                            process.terminate();
+                            self.release_process_id(process_id).await;
+                            return Err(err);
+                        } else {
+                            return Err(err);
+                        }
                     }
                 }
             }

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

@@ -5,6 +5,7 @@ use codex_exec_server::ExecProcess;
 use codex_exec_server::ExecProcessEventReceiver;
 use codex_exec_server::ExecServerError;
 use codex_exec_server::ProcessId;
+use codex_exec_server::ProcessSignal;
 use codex_exec_server::ReadResponse;
 use codex_exec_server::StartedExecProcess;
 use codex_exec_server::WriteResponse;
@@ -63,6 +64,10 @@ impl ExecProcess for MockExecProcess {
         Ok(self.write_response.clone())
     }
 
+    async fn signal(&self, _signal: ProcessSignal) -> Result<(), ExecServerError> {
+        Ok(())
+    }
+
     async fn terminate(&self) -> Result<(), ExecServerError> {
         Ok(())
     }