Migrate tui to use UserTurn (#9497)

- `tui/` and `tui2/` submit `Op::UserTurn` and own full turn context
(cwd/approval/sandbox/model/etc.).
- `Op::UserInput` is documented as legacy in `codex-protocol` (doc-only;
no `#[deprecated]` to avoid `-D warnings` fallout).
- Remove obsolete `#[allow(deprecated)]` and the unused `ConversationId`
alias/re-export.

codex-rs/protocol/src/thread_id.rs

@@ -70,10 +70,6 @@ impl JsonSchema for ThreadId {
     }
 }
 
-/// Backward-compatible alias for the previous name.
-#[deprecated(note = "use ThreadId instead")]
-pub type ConversationId = ThreadId;
-
 #[cfg(test)]
 mod tests {
     use super::*;