Validate CODEX_HOME before resolving (#10249)

Summary
- require `CODEX_HOME` to point to an existing directory before
canonicalizing and surface clear errors otherwise
- share the same helper logic in both `core` and `rmcp-client` and add
unit tests that cover missing, non-directory, valid, and default paths

This addresses #9222

codex-rs/rmcp-client/src/lib.rs

@@ -1,5 +1,4 @@
 mod auth_status;
-mod find_codex_home;
 mod logging_client_handler;
 mod oauth;
 mod perform_oauth_login;