Make the update_plan tool opt-in (#41744)

## What changed

- Default `tools.update_plan.enabled` to `false`; users can explicitly enable it to expose `update_plan`.
- Remove bundled `update_plan` guidance from model, collaboration-mode, multi-agent, compaction, prewarm, and goal-continuation prompts when the tool is disabled.
- Preserve custom base instructions, model catalog instructions, collaboration policies, and user goal text even when they mention planning or `update_plan`.

## Testing

- Cover default and explicitly enabled tool registration, prompt consistency across request paths, and preservation of custom instructions.

GitOrigin-RevId: a53964e6e72f98e2557dd3090fdb7caccb956527

codex-rs/config/src/config_toml.rs

@@ -632,7 +632,7 @@ pub struct ExperimentalRequestUserInput {
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema)]
 #[schemars(deny_unknown_fields)]
 pub struct UpdatePlanToolConfig {
-    #[serde(default = "default_true")]
+    #[serde(default)]
     pub enabled: bool,
 }