[codex] rename rollout budget error to session budget error (#29744)

## Summary

- rename the rollout-budget exhaustion error from
`RolloutBudgetExceeded` to `SessionBudgetExceeded`
- expose the matching app-server v2 wire value as
`sessionBudgetExceeded`
- regenerate JSON/TypeScript schema fixtures and update the app-server
docs and focused tests

This is a naming-only follow-up to #29715 based on [Pavel's review
suggestion](https://github.com/openai/codex/pull/29715#discussion_r3463183480).
Runtime behavior is unchanged.

## Tests

- `just test -p codex-core rollout_budget`
- `just test -p codex-app-server-protocol`
- `just fmt`
- `just write-app-server-schema`

codex-rs/core/src/compact.rs

@@ -275,7 +275,7 @@ async fn run_compact_task_inner_impl(
             Err(err @ (CodexErr::Interrupted | CodexErr::TurnAborted)) => {
                 return Err(err);
             }
-            Err(e @ CodexErr::RolloutBudgetExceeded) => {
+            Err(e @ CodexErr::SessionBudgetExceeded) => {
                 sess.track_turn_codex_error(turn_context.as_ref(), &e);
                 let event = EventMsg::Error(e.to_error_event(/*message_prefix*/ None));
                 sess.send_event(&turn_context, event).await;