[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/error.rs

@@ -70,7 +70,7 @@ pub enum CodexErr {
     TurnAborted,
 
     #[error("shared rollout token budget exhausted")]
-    RolloutBudgetExceeded,
+    SessionBudgetExceeded,
 
     /// Returned by ResponsesClient when the SSE stream disconnects or errors out **after** the HTTP
     /// handshake has succeeded but **before** it finished emitting `response.completed`.
@@ -176,7 +176,7 @@ impl CodexErr {
     pub fn is_retryable(&self) -> bool {
         match self {
             CodexErr::TurnAborted
-            | CodexErr::RolloutBudgetExceeded
+            | CodexErr::SessionBudgetExceeded
             | CodexErr::Interrupted
             | CodexErr::EnvVar(_)
             | CodexErr::Fatal(_)
@@ -224,7 +224,7 @@ impl CodexErr {
     pub fn to_codex_protocol_error(&self) -> CodexErrorInfo {
         match self {
             CodexErr::ContextWindowExceeded => CodexErrorInfo::ContextWindowExceeded,
-            CodexErr::RolloutBudgetExceeded => CodexErrorInfo::RolloutBudgetExceeded,
+            CodexErr::SessionBudgetExceeded => CodexErrorInfo::SessionBudgetExceeded,
             CodexErr::UsageLimitReached(_)
             | CodexErr::QuotaExceeded
             | CodexErr::UsageNotIncluded => CodexErrorInfo::UsageLimitExceeded,