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/compact.rs

@@ -281,7 +281,7 @@ async fn run_compact_task_inner_impl(
         let turn_input_len = turn_input.len();
         let prompt = Prompt {
             input: turn_input,
-            base_instructions: sess.get_base_instructions().await,
+            base_instructions: sess.get_prompt_base_instructions().await,
             ..Default::default()
         };
         let attempt_result = drain_to_completed(