fix: create separate test_support crates to eliminate #[allow(dead_code)] (#1667)

Because of a quirk of how implementation tests work in Rust, we had a
number of `#[allow(dead_code)]` annotations that were misleading because
the functions _were_ being used, just not by all integration tests in a
`tests/` folder, so when compiling the test that did not use the
function, clippy would complain that it was unused.

This fixes things by create a "test_support" crate under the `tests/`
folder that is imported as a dev dependency for the respective crate.

codex-rs/mcp-server/tests/common/lib.rs

@@ -0,0 +1,9 @@
+mod mcp_process;
+mod mock_model_server;
+mod responses;
+
+pub use mcp_process::McpProcess;
+pub use mock_model_server::create_mock_chat_completions_server;
+pub use responses::create_apply_patch_sse_response;
+pub use responses::create_final_assistant_message_sse_response;
+pub use responses::create_shell_sse_response;