Share additional tool declarations across response item clones (#54945)

## Why

Cloning `ResponseItem::AdditionalTools` deep-copies its tool declarations for history and request copies.

## What changed

Store `tools` in an `Arc<[serde_json::Value]>` so clones share immutable declarations. Update constructors and existing test fixtures to use the shared representation.

GitOrigin-RevId: eb8831774afb45fe8431a796f34ae9d1243e884a

codex-rs/protocol/src/models.rs

@@ -2,6 +2,7 @@ use std::collections::HashMap;
 use std::io;
 use std::num::NonZeroUsize;
 use std::path::Path;
+use std::sync::Arc;
 
 use codex_utils_image::PromptImageMode;
 use codex_utils_image::data_url_from_bytes;
@@ -1055,7 +1056,8 @@ pub enum ResponseItem {
         id: Option<ResponseItemId>,
         role: String,
         #[ts(type = "unknown[]")]
-        tools: Vec<serde_json::Value>,
+        // Share immutable declarations across history and request copies.
+        tools: Arc<[serde_json::Value]>,
     },
     Message {
         #[serde(default, skip_serializing_if = "Option::is_none")]
@@ -2807,7 +2809,7 @@ mod tests {
         let mut additional_tools = ResponseItem::AdditionalTools {
             id: None,
             role: "developer".to_string(),
-            tools: Vec::new(),
+            tools: Arc::from([]),
         };
         additional_tools.set_id(Some(ResponseItemId::with_suffix("at", "test")));
         assert_eq!(