feat: search_tool migrate to bring you own tool of Responses API (#14274)

## Why

to support a new bring your own search tool in Responses
API(https://developers.openai.com/api/docs/guides/tools-tool-search#client-executed-tool-search)
we migrating our bm25 search tool to use official way to execute search
on client and communicate additional tools to the model.

## What
- replace the legacy `search_tool_bm25` flow with client-executed
`tool_search`
- add protocol, SSE, history, and normalization support for
`tool_search_call` and `tool_search_output`
- return namespaced Codex Apps search results and wire namespaced
follow-up tool calls back into MCP dispatch

codex-rs/core/src/agent/control.rs

@@ -878,6 +878,7 @@ mod tests {
         let parent_spawn_call = ResponseItem::FunctionCall {
             id: None,
             name: "spawn_agent".to_string(),
+            namespace: None,
             arguments: "{}".to_string(),
             call_id: parent_spawn_call_id.clone(),
         };
@@ -960,6 +961,7 @@ mod tests {
         let parent_spawn_call = ResponseItem::FunctionCall {
             id: None,
             name: "spawn_agent".to_string(),
+            namespace: None,
             arguments: "{}".to_string(),
             call_id: parent_spawn_call_id.clone(),
         };
@@ -1035,6 +1037,7 @@ mod tests {
         let parent_spawn_call = ResponseItem::FunctionCall {
             id: None,
             name: "spawn_agent".to_string(),
+            namespace: None,
             arguments: "{}".to_string(),
             call_id: parent_spawn_call_id.clone(),
         };