feat: change the behavior of SetDefaultModel RPC so None clears the value. (#3529)

It turns out that we want slightly different behavior for the
`SetDefaultModel` RPC because some models do not work with reasoning
(like GPT-4.1), so we should be able to explicitly clear this value.

Verified in `codex-rs/mcp-server/tests/suite/set_default_model.rs`.

codex-rs/protocol/src/mcp_protocol.rs

@@ -424,8 +424,11 @@ pub struct GetUserSavedConfigResponse {
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, TS)]
 #[serde(rename_all = "camelCase")]
 pub struct SetDefaultModelParams {
+    /// If set to None, this means `model` should be cleared in config.toml.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub model: Option<String>,
+    /// If set to None, this means `model_reasoning_effort` should be cleared
+    /// in config.toml.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub reasoning_effort: Option<ReasoningEffort>,
 }