[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/protocol/src/protocol.rs

@@ -1690,7 +1690,7 @@ pub enum NonSteerableTurnKind {
 #[ts(rename_all = "snake_case")]
 pub enum CodexErrorInfo {
     ContextWindowExceeded,
-    RolloutBudgetExceeded,
+    SessionBudgetExceeded,
     UsageLimitExceeded,
     ServerOverloaded,
     CyberPolicy,
@@ -1728,7 +1728,7 @@ impl CodexErrorInfo {
         match self {
             Self::ThreadRollbackFailed | Self::ActiveTurnNotSteerable { .. } => false,
             Self::ContextWindowExceeded
-            | Self::RolloutBudgetExceeded
+            | Self::SessionBudgetExceeded
             | Self::UsageLimitExceeded
             | Self::ServerOverloaded
             | Self::CyberPolicy