[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/process.rs

@@ -10,6 +10,7 @@ use crate::ExecServerError;
 use crate::ProcessId;
 use crate::protocol::ExecParams;
 use crate::protocol::ProcessOutputChunk;
+use crate::protocol::ProcessSignal;
 use crate::protocol::ReadResponse;
 use crate::protocol::WriteResponse;
 
@@ -178,6 +179,8 @@ pub trait ExecProcess: Send + Sync {
 
     async fn write(&self, chunk: Vec<u8>) -> Result<WriteResponse, ExecServerError>;
 
+    async fn signal(&self, signal: ProcessSignal) -> Result<(), ExecServerError>;
+
     async fn terminate(&self) -> Result<(), ExecServerError>;
 }