[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/suite/mod.rs

@@ -2,6 +2,7 @@
 mod auth;
 mod codex_message_processor_flow;
 mod codex_tool;
+mod config;
 mod create_conversation;
 mod interrupt;
 mod login;