feat: added SetDefaultModel to JSON-RPC server (#3512)

This adds `SetDefaultModel`, which takes `model` and `reasoning_effort`
as optional fields. If set, the field will overwrite what is in the
user's `config.toml`.

This reuses logic that was added to support the `/model` command in the
TUI: https://github.com/openai/codex/pull/2799.

codex-rs/core/src/config.rs

@@ -495,7 +495,7 @@ fn apply_toml_override(root: &mut TomlValue, path: &str, value: TomlValue) {
 }
 
 /// Base config deserialized from ~/.codex/config.toml.
-#[derive(Deserialize, Debug, Clone, Default)]
+#[derive(Deserialize, Debug, Clone, Default, PartialEq)]
 pub struct ConfigToml {
     /// Optional override of model selection.
     pub model: Option<String>,
@@ -627,7 +627,7 @@ pub struct ProjectConfig {
     pub trust_level: Option<String>,
 }
 
-#[derive(Deserialize, Debug, Clone, Default)]
+#[derive(Deserialize, Debug, Clone, Default, PartialEq)]
 pub struct ToolsToml {
     #[serde(default, alias = "web_search_request")]
     pub web_search: Option<bool>,