## Why
A pending realtime connection handshake can hold up subsequent attachment requests. Callers need to cancel obsolete attempts or limit how long they may take to establish a connection.
## What changed
- Add `RealtimeAttachmentOptions` with an optional cancellation receiver and `start_before` deadline to `Op::RealtimeConversationAttach`.
- Interrupt pending connection work and finish cleanup before acknowledging cancellation. Reject cancelled or expired requests before replacing an existing conversation, and recheck cancellation and the deadline before publishing a connection.
- Preserve a successfully published connection when cancellation arrives later. Dropping a cancellation sender whose value is `false` lets the attempt continue.
- Prepare the daemon startup test's socket directory with `codex_uds::prepare_private_socket_directory`.
## Testing
Add integration coverage for cancelling a held handshake, skipping cancelled or expired queued requests, preserving the next usable call, and releasing a pending handshake when its deadline expires without a cancellation receiver.
GitOrigin-RevId: 7144c1237bcbe3b859f10b654d919ae9ff20cbbb
Jenia Varavva
9f04d886f3