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

---------

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

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

@@ -46,11 +46,15 @@ impl From<Vec<u8>> for ByteChunk {
 #[serde(rename_all = "camelCase")]
 pub struct InitializeParams {
     pub client_name: String,
+    #[serde(default)]
+    pub resume_session_id: Option<String>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
-pub struct InitializeResponse {}
+pub struct InitializeResponse {
+    pub session_id: String,
+}
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]