[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/analytics/src/facts.rs

@@ -139,7 +139,7 @@ impl TurnCodexErrorFact {
 #[serde(rename_all = "snake_case")]
 pub enum CodexErrKind {
     TurnAborted,
-    RolloutBudgetExceeded,
+    SessionBudgetExceeded,
     Stream,
     ContextWindowExceeded,
     ThreadNotFound,
@@ -196,7 +196,7 @@ impl From<&CodexErr> for CodexErrKind {
     fn from(error: &CodexErr) -> Self {
         match error {
             CodexErr::TurnAborted => CodexErrKind::TurnAborted,
-            CodexErr::RolloutBudgetExceeded => CodexErrKind::RolloutBudgetExceeded,
+            CodexErr::SessionBudgetExceeded => CodexErrKind::SessionBudgetExceeded,
             CodexErr::Stream(..) => CodexErrKind::Stream,
             CodexErr::ContextWindowExceeded => CodexErrKind::ContextWindowExceeded,
             CodexErr::ThreadNotFound(_) => CodexErrKind::ThreadNotFound,