[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/exec-server/src/local_process.rs

@@ -10,6 +10,7 @@ use codex_protocol::config_types::EnvironmentVariablePattern;
 use codex_protocol::config_types::ShellEnvironmentPolicy;
 use codex_protocol::shell_environment;
 use codex_utils_pty::ExecCommandSession;
+use codex_utils_pty::ProcessSignal as PtyProcessSignal;
 use codex_utils_pty::TerminalSize;
 use tokio::sync::Mutex;
 use tokio::sync::Notify;
@@ -33,8 +34,11 @@ use crate::protocol::ExecOutputStream;
 use crate::protocol::ExecParams;
 use crate::protocol::ExecResponse;
 use crate::protocol::ProcessOutputChunk;
+use crate::protocol::ProcessSignal;
 use crate::protocol::ReadParams;
 use crate::protocol::ReadResponse;
+use crate::protocol::SignalParams;
+use crate::protocol::SignalResponse;
 use crate::protocol::TerminateParams;
 use crate::protocol::TerminateResponse;
 use crate::protocol::WriteParams;
@@ -272,7 +276,6 @@ impl LocalProcess {
         &self,
         params: ReadParams,
     ) -> Result<ReadResponse, JSONRPCErrorError> {
-        let _process_id = params.process_id.clone();
         let after_seq = params.after_seq.unwrap_or(0);
         let max_bytes = params.max_bytes.unwrap_or(usize::MAX);
         let wait = Duration::from_millis(params.wait_ms.unwrap_or(0));
@@ -351,7 +354,6 @@ impl LocalProcess {
         &self,
         params: WriteParams,
     ) -> Result<WriteResponse, JSONRPCErrorError> {
-        let _process_id = params.process_id.clone();
         let _input_bytes = params.chunk.0.len();
         let writer_tx = {
             let process_map = self.inner.processes.lock().await;
@@ -383,11 +385,33 @@ impl LocalProcess {
         })
     }
 
+    pub(crate) async fn signal_process(
+        &self,
+        params: SignalParams,
+    ) -> Result<SignalResponse, JSONRPCErrorError> {
+        {
+            let process_map = self.inner.processes.lock().await;
+            match process_map.get(&params.process_id) {
+                Some(ProcessEntry::Running(process)) => {
+                    if process.exit_code.is_some() {
+                        return Ok(SignalResponse {});
+                    }
+                    process
+                        .session
+                        .signal(pty_process_signal(params.signal))
+                        .map_err(|err| internal_error(format!("failed to signal process: {err}")))?
+                }
+                Some(ProcessEntry::Starting) | None => {}
+            }
+        }
+
+        Ok(SignalResponse {})
+    }
+
     pub(crate) async fn terminate_process(
         &self,
         params: TerminateParams,
     ) -> Result<TerminateResponse, JSONRPCErrorError> {
-        let _process_id = params.process_id.clone();
         let running = {
             let process_map = self.inner.processes.lock().await;
             match process_map.get(&params.process_id) {
@@ -483,6 +507,10 @@ impl ExecProcess for LocalExecProcess {
         self.backend.write(&self.process_id, chunk).await
     }
 
+    async fn signal(&self, signal: ProcessSignal) -> Result<(), ExecServerError> {
+        self.backend.signal(&self.process_id, signal).await
+    }
+
     async fn terminate(&self) -> Result<(), ExecServerError> {
         self.backend.terminate(&self.process_id).await
     }
@@ -519,6 +547,20 @@ impl LocalProcess {
         .map_err(map_handler_error)
     }
 
+    async fn signal(
+        &self,
+        process_id: &ProcessId,
+        signal: ProcessSignal,
+    ) -> Result<(), ExecServerError> {
+        self.signal_process(SignalParams {
+            process_id: process_id.clone(),
+            signal,
+        })
+        .await
+        .map_err(map_handler_error)?;
+        Ok(())
+    }
+
     async fn terminate(&self, process_id: &ProcessId) -> Result<(), ExecServerError> {
         self.terminate_process(TerminateParams {
             process_id: process_id.clone(),
@@ -529,6 +571,12 @@ impl LocalProcess {
     }
 }
 
+fn pty_process_signal(signal: ProcessSignal) -> PtyProcessSignal {
+    match signal {
+        ProcessSignal::Interrupt => PtyProcessSignal::Interrupt,
+    }
+}
+
 fn map_handler_error(error: JSONRPCErrorError) -> ExecServerError {
     ExecServerError::Server {
         code: error.code,