Allow disabling the update_plan tool (#35054)

## What changed

- Add a default-on `tools.update_plan.enabled` configuration option.
- Omit `update_plan` from the visible and registered tool sets when the option is disabled.

## Testing

- Cover configuration resolution and tool registration for the disabled setting.

GitOrigin-RevId: c13aa463a6911956fca9f0ef5b74841b543798c4

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

@@ -1030,6 +1030,9 @@ pub struct Config {
     /// Whether to register the experimental request_user_input tool.
     pub experimental_request_user_input_enabled: bool,
 
+    /// Whether to register the update_plan tool.
+    pub update_plan_enabled: bool,
+
     /// Configuration for the experimental code-mode tool surface.
     pub code_mode: CodeModeConfig,
 
@@ -2537,6 +2540,14 @@ fn resolve_experimental_request_user_input_enabled(config_toml: &ConfigToml) ->
         .is_none_or(|config| config.enabled)
 }
 
+fn resolve_update_plan_enabled(config_toml: &ConfigToml) -> bool {
+    config_toml
+        .tools
+        .as_ref()
+        .and_then(|tools| tools.update_plan.as_ref())
+        .is_none_or(|config| config.enabled)
+}
+
 fn resolve_orchestrator_feature_enabled(
     feature: Option<&codex_config::config_toml::OrchestratorFeatureToml>,
 ) -> bool {
@@ -3577,6 +3588,7 @@ impl Config {
         let web_search_config = resolve_web_search_config(&cfg);
         let experimental_request_user_input_enabled =
             resolve_experimental_request_user_input_enabled(&cfg);
+        let update_plan_enabled = resolve_update_plan_enabled(&cfg);
         let code_mode = resolve_code_mode_config(&cfg);
         let multi_agent_v2 = resolve_multi_agent_v2_config(&cfg);
         let token_budget = resolve_token_budget_config(&cfg, &features)?;
@@ -4108,6 +4120,7 @@ impl Config {
             web_search_mode: constrained_web_search_mode.value,
             web_search_config,
             experimental_request_user_input_enabled,
+            update_plan_enabled,
             code_mode,
             use_experimental_unified_exec_tool,
             background_terminal_max_timeout,