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/turn.rs

@@ -1370,7 +1370,7 @@ async fn run_sampling_request(
     cancellation_token: CancellationToken,
 ) -> CodexResult<(SamplingRequestResult, Vec<ResponseItem>)> {
     let turn_context = Arc::clone(&step_context.turn);
-    let base_instructions = sess.get_base_instructions().await;
+    let base_instructions = sess.get_prompt_base_instructions().await;
 
     let tool_runtime = ToolCallRuntime::new(
         Arc::clone(&sess),