Rename the history notes extension config option (#39830)

## What changed

Rename the token-budget configuration key and its Rust field from
`use_history_notes_history` to `use_history_notes_extension`. Update the config
schema, extension gating, explicit-setting detection, and tests to use the new
name consistently.

GitOrigin-RevId: b6a514e23f02095fcd1d35dc42eeb7394b800c77

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

@@ -654,7 +654,7 @@ async fn load_config_resolves_token_budget_config() -> std::io::Result<()> {
             r#"
 [features.token_budget]
 enabled = true
-use_history_notes_history = true
+use_history_notes_extension = true
 reminder_threshold_tokens = 16000
 reminder_message_template = "Custom reminder: {n_remaining} tokens."
 guidance_message = "Preserve important state before compaction."
@@ -662,7 +662,7 @@ auto_compact_fallback_prompt = "  Write notes immediately.  "
 auto_compact_fallback_buffer_tokens = 8000
 "#,
             TokenBudgetConfig {
-                use_history_notes_history: true,
+                use_history_notes_extension: true,
                 reminder_threshold_tokens: Some(16_000),
                 reminder_message_template: "Custom reminder: {n_remaining} tokens.".to_string(),
                 guidance_message: Some("Preserve important state before compaction.".to_string()),

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

@@ -1087,7 +1087,7 @@ const AUTO_COMPACT_FALLBACK_PROMPT_MAX_BYTES: usize = 2000;
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize)]
 pub struct TokenBudgetConfig {
-    pub use_history_notes_history: bool,
+    pub use_history_notes_extension: bool,
     pub reminder_threshold_tokens: Option<i64>,
     pub reminder_message_template: String,
     pub guidance_message: Option<String>,
@@ -1180,7 +1180,7 @@ impl TokenBudgetConfig {
 impl Default for TokenBudgetConfig {
     fn default() -> Self {
         Self {
-            use_history_notes_history: false,
+            use_history_notes_extension: false,
             reminder_threshold_tokens: None,
             reminder_message_template: DEFAULT_TOKEN_BUDGET_REMINDER_MESSAGE_TEMPLATE.to_string(),
             guidance_message: None,
@@ -2720,8 +2720,8 @@ fn resolve_token_budget_config(
     }
 
     let token_budget_config = token_budget_toml_config(config_toml.features.as_ref());
-    let use_history_notes_history = token_budget_config
-        .and_then(|config| config.use_history_notes_history)
+    let use_history_notes_extension = token_budget_config
+        .and_then(|config| config.use_history_notes_extension)
         .unwrap_or_default();
     let reminder_threshold_tokens =
         token_budget_config.and_then(|config| config.reminder_threshold_tokens);
@@ -2740,7 +2740,7 @@ fn resolve_token_budget_config(
         token_budget_config.and_then(|config| config.auto_compact_fallback_buffer_tokens);
 
     let token_budget = TokenBudgetConfig {
-        use_history_notes_history,
+        use_history_notes_extension,
         reminder_threshold_tokens,
         reminder_message_template,
         guidance_message,