Allow models to enable token budgeting by default (#41803)

## What changed

- Add model metadata flags for enabling token budgeting and its history-notes extension.
- Apply those activation defaults once at thread startup when the user has not explicitly configured token budgeting, while respecting managed feature requirements.
- Continue resolving model-owned prompts for the active model so model switches can use their own token-budget guidance.

## Testing

- Update the token-budget integration test to verify that model metadata activates the feature without manual configuration.

GitOrigin-RevId: 73a32623eaeabb9243e240c43da7a5e7e93e9a19

codex-rs/protocol/src/openai_models.rs

@@ -594,6 +594,10 @@ pub struct ToolMessage {
 /// Model-owned defaults for the context-window token-budget feature.
 #[derive(Debug, Serialize, Deserialize, Clone, PartialEq, Eq, TS, JsonSchema)]
 pub struct ModelTokenBudgetConfig {
+    #[serde(default)]
+    pub enabled: bool,
+    #[serde(default)]
+    pub use_history_notes_extension: bool,
     pub reminder_threshold_tokens: i64,
     pub reminder_message_template: String,
     pub guidance_message: String,