Add a fallback phase before automatic context rollover (#33255)

## Why

Token-budget sessions need a final opportunity to preserve important state before the current context is compacted.

## What changed

- When `auto_compact_fallback_prompt` is configured, reserve `auto_compact_fallback_buffer_tokens` beyond the base auto-compaction limit.
- At the base limit, inject the fallback prompt once as a developer message while keeping the normal tool surface available. Continue reporting zero base-window tokens during this reserve.
- Roll over when the reserve or full model context is exhausted, and skip the fallback when a new context was already requested.

## Testing

Added coverage for using the fallback reserve, rolling over after it is exhausted, and bypassing the fallback for an explicit `new_context` request.

GitOrigin-RevId: 32983ecadc5cb4927e15ecd7b1e896cb52a9ad3b

codex-rs/core/src/config/mod.rs

@@ -1154,6 +1154,16 @@ pub struct TokenBudgetConfig {
     pub auto_compact_fallback_buffer_tokens: Option<i64>,
 }
 
+impl TokenBudgetConfig {
+    pub(crate) fn fallback_buffer_tokens(&self) -> i64 {
+        if self.auto_compact_fallback_prompt.is_some() {
+            self.auto_compact_fallback_buffer_tokens.unwrap_or(0)
+        } else {
+            0
+        }
+    }
+}
+
 impl Default for TokenBudgetConfig {
     fn default() -> Self {
         Self {