[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/session/rollout_budget.rs

@@ -30,7 +30,7 @@ impl Session {
             .rollout_budget()
             .record_usage(usage)
         {
-            return Err(CodexErr::RolloutBudgetExceeded);
+            return Err(CodexErr::SessionBudgetExceeded);
         }
         Ok(())
     }