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/Cargo.toml

@@ -40,6 +40,7 @@ tokio = { workspace = true, features = [
 ] }
 tokio-tungstenite = { workspace = true }
 tracing = { workspace = true }
+uuid = { workspace = true, features = ["v4"] }
 
 [dev-dependencies]
 anyhow = { workspace = true }