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/interrupt.rs

@@ -1,5 +1,5 @@
 #![cfg(unix)]
-mod common;
+// Support code lives in the `mcp_test_support` crate under tests/common.
 
 use std::path::Path;
 
@@ -11,9 +11,9 @@ use serde_json::json;
 use tempfile::TempDir;
 use tokio::time::timeout;
 
-use crate::common::McpProcess;
-use crate::common::create_mock_chat_completions_server;
-use crate::common::create_shell_sse_response;
+use mcp_test_support::McpProcess;
+use mcp_test_support::create_mock_chat_completions_server;
+use mcp_test_support::create_shell_sse_response;
 
 const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(10);