[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/tests/suite/rollout_budget.rs

@@ -252,7 +252,7 @@ async fn exhausted_budget_fails_current_and_later_turns() -> Result<()> {
             matches!(
                 event,
                 EventMsg::Error(error)
-                    if error.codex_error_info == Some(CodexErrorInfo::RolloutBudgetExceeded)
+                    if error.codex_error_info == Some(CodexErrorInfo::SessionBudgetExceeded)
             )
         })
         .await;
@@ -315,7 +315,7 @@ async fn compaction_budget_exhaustion_fails_without_retry(remote_v2: bool) -> Re
         matches!(
             event,
             EventMsg::Error(error)
-                if error.codex_error_info == Some(CodexErrorInfo::RolloutBudgetExceeded)
+                if error.codex_error_info == Some(CodexErrorInfo::SessionBudgetExceeded)
         )
     })
     .await;