Make user_agent optional (#3436)

# External (non-OpenAI) Pull Request Requirements

Currently, mcp server fail to start with:
```
🖐  MCP client for `<CLIENT>` failed to start: missing field `user_agent`
````

It isn't clear to me yet why this is happening. My understanding is that
this struct is simply added as a new field to the response but this
should fix it until I figure out the full story here.

<img width="714" height="262" alt="CleanShot 2025-09-10 at 13 58 59"
src="https://github.com/user-attachments/assets/946b1313-5c1c-43d3-8ae8-ecc3de3406fc"
/>

codex-rs/mcp-types/src/lib.rs

@@ -496,7 +496,8 @@ pub struct McpServerInfo {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub title: Option<String>,
     pub version: String,
-    pub user_agent: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub user_agent: Option<String>,
 }
 
 #[derive(Debug, Clone, PartialEq, Deserialize, Serialize, TS)]