[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/protocol-ts/src/lib.rs

@@ -46,6 +46,7 @@ pub fn generate_ts(out_dir: &Path, prettier: Option<&Path>) -> Result<()> {
     codex_protocol::mcp_protocol::ApplyPatchApprovalResponse::export_all_to(out_dir)?;
     codex_protocol::mcp_protocol::ExecCommandApprovalParams::export_all_to(out_dir)?;
     codex_protocol::mcp_protocol::ExecCommandApprovalResponse::export_all_to(out_dir)?;
+    codex_protocol::mcp_protocol::GetUserSavedConfigResponse::export_all_to(out_dir)?;
     codex_protocol::mcp_protocol::ServerNotification::export_all_to(out_dir)?;
 
     generate_index_ts(out_dir)?;