[mcp-server] Add GetConfig endpoint (#2725)

## Summary
Adds a GetConfig request to the MCP Protocol, so MCP clients can
evaluate the resolved config.toml settings which the harness is using.

## Testing
- [x] Added an end to end test of the endpoint

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

@@ -228,6 +228,11 @@ 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 `loginChatGpt` JSON-RPC request.
     pub async fn send_login_chat_gpt_request(&mut self) -> anyhow::Result<i64> {
         self.send_request("loginChatGpt", None).await