Move responses mocking helpers to a shared lib (#3878)

These are generally useful

codex-rs/core/tests/common/lib.rs

@@ -7,6 +7,8 @@ use codex_core::config::Config;
 use codex_core::config::ConfigOverrides;
 use codex_core::config::ConfigToml;
 
+pub mod responses;
+
 /// Returns a default `Config` whose on-disk state is confined to the provided
 /// temporary directory. Using a per-test directory keeps tests hermetic and
 /// avoids clobbering a developer’s real `~/.codex`.