[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/protocol.rs

@@ -15,6 +15,7 @@ pub const INITIALIZED_METHOD: &str = "initialized";
 pub const EXEC_METHOD: &str = "process/start";
 pub const EXEC_READ_METHOD: &str = "process/read";
 pub const EXEC_WRITE_METHOD: &str = "process/write";
+pub const EXEC_SIGNAL_METHOD: &str = "process/signal";
 pub const EXEC_TERMINATE_METHOD: &str = "process/terminate";
 pub const EXEC_OUTPUT_DELTA_METHOD: &str = "process/output";
 pub const EXEC_EXITED_METHOD: &str = "process/exited";
@@ -166,6 +167,23 @@ pub struct WriteResponse {
     pub status: WriteStatus,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub enum ProcessSignal {
+    Interrupt,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SignalParams {
+    pub process_id: ProcessId,
+    pub signal: ProcessSignal,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SignalResponse {}
+
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct TerminateParams {