Add auto-compaction fallback token-budget settings (#33243)

## What changed

- Add `auto_compact_fallback_prompt` and
  `auto_compact_fallback_buffer_tokens` to `features.token_budget` and the
  generated configuration schema.
- Trim empty fallback prompts, limit prompts to 2,000 bytes, require a buffer
  when a prompt is configured, and reject non-positive buffer values.
- Preserve the new settings when locking resolved session configuration.

## Testing

- Cover config resolution and validation for overlong prompts, missing
  buffers, and non-positive buffers.
- Extend the configuration-lock test to cover both settings.

GitOrigin-RevId: 6463f963ba1dbd633d76601b8344360a8c85cf8b

codex-rs/features/src/feature_configs.rs

@@ -98,6 +98,14 @@ pub struct TokenBudgetConfigToml {
     #[serde(skip_serializing_if = "Option::is_none")]
     #[schemars(length(max = 2000))]
     pub guidance_message: Option<String>,
+    /// Developer message sampled before an automatic context-window rollover.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    #[schemars(length(max = 2000))]
+    pub auto_compact_fallback_prompt: Option<String>,
+    /// Additional tokens available after the compaction threshold for fallback note-taking.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    #[schemars(range(min = 1))]
+    pub auto_compact_fallback_buffer_tokens: Option<i64>,
 }
 
 impl FeatureConfig for TokenBudgetConfigToml {