feat: exec-server prep for unified exec (#15691)

This PR partially rebase `unified_exec` on the `exec-server` and adapt
the `exec-server` accordingly.

## What changed in `exec-server`

1. Replaced the old "broadcast-driven; process-global" event model with
process-scoped session events. The goal is to be able to have dedicated
handler for each process.
2. Add to protocol contract to support explicit lifecycle status and
stream ordering:
- `WriteResponse` now returns `WriteStatus` (Accepted, UnknownProcess,
StdinClosed, Starting) instead of a bool.
  - Added seq fields to output/exited notifications.
  - Added terminal process/closed notification.
3. Demultiplexed remote notifications into per-process channels. Same as
for the event sys
4. Local and remote backends now both implement ExecBackend.
5. Local backend wraps internal process ID/operations into per-process
ExecProcess objects.
6. Remote backend registers a session channel before launch and
unregisters on failed launch.

## What changed in `unified_exec`

1. Added unified process-state model and backend-neutral process
wrapper. This will probably disappear in the future, but it makes it
easier to keep the work flowing on both side.
- `UnifiedExecProcess` now handles both local PTY sessions and remote
exec-server processes through a shared `ProcessHandle`.
- Added `ProcessState` to track has_exited, exit_code, and terminal
failure message consistently across backends.
2. Routed write and lifecycle handling through process-level methods.

## Some rationals

1. The change centralizes execution transport in exec-server while
preserving policy and orchestration ownership in core, avoiding
duplicated launch approval logic. This comes from internal discussion.
2. Session-scoped events remove coupling/cross-talk between processes
and make stream ordering and terminal state explicit (seq, closed,
failed).
3. The failure-path surfacing (remote launch failures, write failures,
transport disconnects) makes command tool output and cleanup behavior
deterministic

## Follow-ups:
* Unify the concept of thread ID behind an obfuscated struct
* FD handling
* Full zsh-fork compatibility
* Full network sandboxing compatibility
* Handle ws disconnection

codex-rs/exec-server/src/process.rs

@@ -1,35 +1,77 @@
+use std::fmt;
+use std::ops::Deref;
+use std::sync::Arc;
+
 use async_trait::async_trait;
-use tokio::sync::broadcast;
+use tokio::sync::watch;
 
 use crate::ExecServerError;
-use crate::protocol::ExecExitedNotification;
-use crate::protocol::ExecOutputDeltaNotification;
 use crate::protocol::ExecParams;
-use crate::protocol::ExecResponse;
-use crate::protocol::ReadParams;
 use crate::protocol::ReadResponse;
-use crate::protocol::TerminateResponse;
 use crate::protocol::WriteResponse;
 
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub enum ExecServerEvent {
-    OutputDelta(ExecOutputDeltaNotification),
-    Exited(ExecExitedNotification),
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct ProcessId(String);
+
+pub struct StartedExecProcess {
+    pub process: Arc<dyn ExecProcess>,
+}
+
+impl ProcessId {
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+
+    pub fn into_inner(self) -> String {
+        self.0
+    }
+}
+
+impl Deref for ProcessId {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        self.as_str()
+    }
+}
+
+impl AsRef<str> for ProcessId {
+    fn as_ref(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl fmt::Display for ProcessId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+impl From<String> for ProcessId {
+    fn from(value: String) -> Self {
+        Self(value)
+    }
 }
 
 #[async_trait]
 pub trait ExecProcess: Send + Sync {
-    async fn start(&self, params: ExecParams) -> Result<ExecResponse, ExecServerError>;
+    fn process_id(&self) -> &ProcessId;
 
-    async fn read(&self, params: ReadParams) -> Result<ReadResponse, ExecServerError>;
+    fn subscribe_wake(&self) -> watch::Receiver<u64>;
 
-    async fn write(
+    async fn read(
         &self,
-        process_id: &str,
-        chunk: Vec<u8>,
-    ) -> Result<WriteResponse, ExecServerError>;
+        after_seq: Option<u64>,
+        max_bytes: Option<usize>,
+        wait_ms: Option<u64>,
+    ) -> Result<ReadResponse, ExecServerError>;
 
-    async fn terminate(&self, process_id: &str) -> Result<TerminateResponse, ExecServerError>;
+    async fn write(&self, chunk: Vec<u8>) -> Result<WriteResponse, ExecServerError>;
 
-    fn subscribe_events(&self) -> broadcast::Receiver<ExecServerEvent>;
+    async fn terminate(&self) -> Result<(), ExecServerError>;
+}
+
+#[async_trait]
+pub trait ExecBackend: Send + Sync {
+    async fn start(&self, params: ExecParams) -> Result<StartedExecProcess, ExecServerError>;
 }