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/core/src/session/mod.rs

@@ -1314,6 +1314,26 @@ impl Session {
         }
     }
 
+    /// Render the request copy without changing instructions persisted or inherited by forks.
+    pub(crate) async fn get_prompt_base_instructions(&self) -> BaseInstructions {
+        let config = self.get_config().await;
+        let instructions = self.get_base_instructions().await;
+        if !config.update_plan_enabled
+            && config.model_catalog.is_none()
+            && matches!(
+                instructions.provenance,
+                Some(BaseInstructionsProvenance::Model { .. })
+            )
+        {
+            BaseInstructions {
+                text: crate::context::without_update_plan_instructions(&instructions.text),
+                ..instructions
+            }
+        } else {
+            instructions
+        }
+    }
+
     // Merges connector IDs into the session-level explicit connector selection.
     #[tracing::instrument(
         level = "trace",