[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/app-server-protocol/src/protocol/v2/shared.rs

@@ -70,7 +70,7 @@ pub enum NonSteerableTurnKind {
 #[ts(export_to = "v2/")]
 pub enum CodexErrorInfo {
     ContextWindowExceeded,
-    RolloutBudgetExceeded,
+    SessionBudgetExceeded,
     UsageLimitExceeded,
     ServerOverloaded,
     CyberPolicy,
@@ -116,7 +116,7 @@ impl From<CoreCodexErrorInfo> for CodexErrorInfo {
     fn from(value: CoreCodexErrorInfo) -> Self {
         match value {
             CoreCodexErrorInfo::ContextWindowExceeded => CodexErrorInfo::ContextWindowExceeded,
-            CoreCodexErrorInfo::RolloutBudgetExceeded => CodexErrorInfo::RolloutBudgetExceeded,
+            CoreCodexErrorInfo::SessionBudgetExceeded => CodexErrorInfo::SessionBudgetExceeded,
             CoreCodexErrorInfo::UsageLimitExceeded => CodexErrorInfo::UsageLimitExceeded,
             CoreCodexErrorInfo::ServerOverloaded => CodexErrorInfo::ServerOverloaded,
             CoreCodexErrorInfo::CyberPolicy => CodexErrorInfo::CyberPolicy,