[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/client.rs

@@ -35,6 +35,7 @@ use crate::protocol::EXEC_EXITED_METHOD;
 use crate::protocol::EXEC_METHOD;
 use crate::protocol::EXEC_OUTPUT_DELTA_METHOD;
 use crate::protocol::EXEC_READ_METHOD;
+use crate::protocol::EXEC_SIGNAL_METHOD;
 use crate::protocol::EXEC_TERMINATE_METHOD;
 use crate::protocol::EXEC_WRITE_METHOD;
 use crate::protocol::EnvironmentInfo;
@@ -80,8 +81,11 @@ use crate::protocol::INITIALIZED_METHOD;
 use crate::protocol::InitializeParams;
 use crate::protocol::InitializeResponse;
 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;
@@ -394,6 +398,23 @@ impl ExecServerClient {
         .await
     }
 
+    pub async fn signal(
+        &self,
+        process_id: &ProcessId,
+        signal: ProcessSignal,
+    ) -> Result<(), ExecServerError> {
+        let _response: SignalResponse = self
+            .call(
+                EXEC_SIGNAL_METHOD,
+                &SignalParams {
+                    process_id: process_id.clone(),
+                    signal,
+                },
+            )
+            .await?;
+        Ok(())
+    }
+
     pub async fn terminate(
         &self,
         process_id: &ProcessId,
@@ -763,6 +784,10 @@ impl Session {
         self.client.write(&self.process_id, chunk).await
     }
 
+    pub(crate) async fn signal(&self, signal: ProcessSignal) -> Result<(), ExecServerError> {
+        self.client.signal(&self.process_id, signal).await
+    }
+
     pub(crate) async fn terminate(&self) -> Result<(), ExecServerError> {
         self.client.terminate(&self.process_id).await?;
         Ok(())