[codex] Use input items for Responses Lite tools (#27946)

When using Responses Lite, we should all use `additional_tools` and a
developer item instead of the top level tools array & instructions
field. This keeps things 1-to-1.

Forced namespacing for _all_ tools will land in a following PR after
some coordination & fixes in Responses API (around collisions & return
items).

The goal is to eventually expand the scope of this to _all_ requests
from codex, but that will require larger coordination across providers &
slower rollout.

codex-rs/protocol/src/models.rs

@@ -932,6 +932,14 @@ impl InternalChatMessageMetadataPassthrough {
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, JsonSchema, TS)]
 #[serde(tag = "type", rename_all = "snake_case")]
 pub enum ResponseItem {
+    #[schemars(skip)]
+    #[ts(skip)]
+    AdditionalTools {
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        id: Option<String>,
+        role: String,
+        tools: Vec<serde_json::Value>,
+    },
     Message {
         #[serde(default, skip_serializing_if = "Option::is_none")]
         #[ts(optional)]
@@ -1160,7 +1168,8 @@ impl ResponseItem {
     /// Returns the non-empty Responses API item ID, if present.
     pub fn id(&self) -> Option<&str> {
         match self {
-            Self::Message { id, .. }
+            Self::AdditionalTools { id, .. }
+            | Self::Message { id, .. }
             | Self::AgentMessage { id, .. }
             | Self::LocalShellCall { id, .. }
             | Self::FunctionCall { id, .. }
@@ -1181,7 +1190,8 @@ impl ResponseItem {
     /// Sets or clears the Responses API item ID for variants that carry one.
     pub fn set_id(&mut self, new_id: Option<String>) {
         match self {
-            Self::Message { id, .. }
+            Self::AdditionalTools { id, .. }
+            | Self::Message { id, .. }
             | Self::AgentMessage { id, .. }
             | Self::LocalShellCall { id, .. }
             | Self::FunctionCall { id, .. }
@@ -1282,7 +1292,7 @@ impl ResponseItem {
                 internal_chat_message_metadata_passthrough: metadata,
                 ..
             } => metadata.as_ref(),
-            Self::CompactionTrigger { .. } | Self::Other => None,
+            Self::CompactionTrigger { .. } | Self::AdditionalTools { .. } | Self::Other => None,
         }
     }
 
@@ -1346,7 +1356,7 @@ impl ResponseItem {
                 internal_chat_message_metadata_passthrough: metadata,
                 ..
             } => Some(metadata),
-            Self::CompactionTrigger { .. } | Self::Other => None,
+            Self::CompactionTrigger { .. } | Self::AdditionalTools { .. } | Self::Other => None,
         }
     }
 }
@@ -2237,6 +2247,14 @@ mod tests {
         item.set_id(/*new_id*/ None);
 
         assert_eq!(item.id(), None);
+
+        let mut additional_tools = ResponseItem::AdditionalTools {
+            id: None,
+            role: "developer".to_string(),
+            tools: Vec::new(),
+        };
+        additional_tools.set_id(Some("at_test".to_string()));
+        assert_eq!(additional_tools.id(), Some("at_test"));
     }
 
     fn response_item_with_passthrough_metadata(