feat: update NewConversationParams to take an optional model_provider (#5793)

An AppServer client should be able to use any (`model_provider`, `model`) in the user's config. `NewConversationParams` already supported specifying the `model`, but this PR expands it to support `model_provider`, as well.

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[//]: # (BEGIN SAPLING FOOTER)
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* #5803
* __->__ #5793

codex-rs/app-server-protocol/src/protocol.rs

@@ -266,6 +266,10 @@ pub struct NewConversationParams {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub model: Option<String>,
 
+    /// Override the model provider to use for this session.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub model_provider: Option<String>,
+
     /// Configuration profile from config.toml to specify default options.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub profile: Option<String>,
@@ -1032,6 +1036,7 @@ mod tests {
             request_id: RequestId::Integer(42),
             params: NewConversationParams {
                 model: Some("gpt-5-codex".to_string()),
+                model_provider: None,
                 profile: None,
                 cwd: None,
                 approval_policy: Some(AskForApproval::OnRequest),