[mcp] Support MCP Apps part 1. (#16082)

- [x] Add `mcpResource/read` method to read mcp resource.

codex-rs/app-server/tests/suite/v2/mod.rs

@@ -14,6 +14,7 @@ mod experimental_api;
 mod experimental_feature_list;
 mod fs;
 mod initialize;
+mod mcp_resource;
 mod mcp_server_elicitation;
 mod mcp_server_status;
 mod model_list;