[mcp-server] Update read config interface (#3093)

## Summary
Follow-up to #3056

This PR updates the mcp-server interface for reading the config settings
saved by the user. At risk of introducing _another_ Config struct, I
think it makes sense to avoid tying our protocol to ConfigToml, as its
become a bit unwieldy. GetConfigTomlResponse was a de-facto struct for
this already - better to make it explicit, in my opinion.

This is technically a breaking change of the mcp-server protocol, but
given the previous interface was introduced so recently in #2725, and we
have not yet even started to call it, I propose proceeding with the
breaking change - but am open to preserving the old endpoint.

## Testing
- [x] Added additional integration test coverage

codex-rs/mcp-server/tests/common/mcp_process.rs

@@ -240,9 +240,9 @@ impl McpProcess {
         self.send_request("getAuthStatus", params).await
     }
 
-    /// Send a `getConfigToml` JSON-RPC request.
-    pub async fn send_get_config_toml_request(&mut self) -> anyhow::Result<i64> {
-        self.send_request("getConfigToml", None).await
+    /// Send a `getUserSavedConfig` JSON-RPC request.
+    pub async fn send_get_user_saved_config_request(&mut self) -> anyhow::Result<i64> {
+        self.send_request("getUserSavedConfig", None).await
     }
 
     /// Send a `loginChatGpt` JSON-RPC request.