chore(api): deprecate MCP connector_id (#3894)

## Summary

Updates MCP tool documentation with deprecation guidance for models
released after September 1, 2026.

## Changes

- Document the connector_id deprecation in Responses, beta Responses,
and Realtime MCP tool types.
- Recommend server_url for remote MCP servers or tunnel_id for Secure
MCP Tunnels.
- Keep the existing connector field, supported values, and
request/response behavior available.