feat: move exec-server ownership (#16344)

This introduces session-scoped ownership for exec-server so ws
disconnects no longer immediately kill running remote exec processes,
and it prepares the protocol for reconnect-based resume.
- add session_id / resume_session_id to the exec-server initialize
handshake
  - move process ownership under a shared session registry
- detach sessions on websocket disconnect and expire them after a TTL
instead of killing processes immediately (we will resume based on this)
- allow a new connection to resume an existing session and take over
notifications/ownership
- I use UUID to make them not predictable as we don't have auth for now
- make detached-session expiry authoritative at resume time so teardown
wins at the TTL boundary
- reject long-poll process/read calls that get resumed out from under an
older attachment

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Co-authored-by: Codex <noreply@openai.com>

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

@@ -5,6 +5,7 @@ use std::time::Duration;
 pub struct ExecServerClientConnectOptions {
     pub client_name: String,
     pub initialize_timeout: Duration,
+    pub resume_session_id: Option<String>,
 }
 
 /// WebSocket connection arguments for a remote exec-server.
@@ -14,4 +15,5 @@ pub struct RemoteExecServerConnectArgs {
     pub client_name: String,
     pub connect_timeout: Duration,
     pub initialize_timeout: Duration,
+    pub resume_session_id: Option<String>,
 }