fix: prevent tool deferral when tool_search is not in request (#311946)
When a custom agent restricts tools to only MCP tools, tool_search gets
filtered out but toolSearchEnabled remained true. This caused all tools
to get defer_loading=true, which Anthropic rejects with 'At least one
tool must have defer_loading=false'.
Add a defense-in-depth check in both Messages API and Responses API:
verify tool_search is actually present in the request's tool list before
enabling tool deferral. This aligns with the BYOK path which already
had this guard.
a9c0f03fb7