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/responses.rs

@@ -39,8 +39,6 @@ pub fn create_shell_sse_response(
     Ok(sse)
 }
 
-// allow dead code
-#[allow(dead_code)]
 pub fn create_final_assistant_message_sse_response(message: &str) -> anyhow::Result<String> {
     let assistant_message = json!({
         "choices": [
@@ -60,8 +58,6 @@ pub fn create_final_assistant_message_sse_response(message: &str) -> anyhow::Res
     Ok(sse)
 }
 
-// allow dead code
-#[allow(dead_code)]
 pub fn create_apply_patch_sse_response(
     patch_content: &str,
     call_id: &str,