Fix the issue #7416 that the codex-cli produce an error "MCP startup
failure on missing 'type' field" in the startup.
- Cause: serde in `convert_to_rmcp`
(`codex-rs/rmcp-client/src/utils.rs`) failed because no `r#type` value
was provided
- Fix: set a default `r#type` value in the corresponding structs
