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/thread_manager.rs

@@ -721,6 +721,7 @@ mod tests {
                 id: None,
                 call_id: "c1".to_string(),
                 name: "tool".to_string(),
+                namespace: None,
                 arguments: "{}".to_string(),
             },
             assistant_msg("a4"),