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/mcp_process.rs

@@ -191,8 +191,6 @@ impl McpProcess {
         Ok(request_id)
     }
 
-    // allow dead code
-    #[allow(dead_code)]
     pub async fn send_response(
         &mut self,
         id: RequestId,
@@ -220,8 +218,6 @@ impl McpProcess {
         let message = serde_json::from_str::<JSONRPCMessage>(&line)?;
         Ok(message)
     }
-    // allow dead code
-    #[allow(dead_code)]
     pub async fn read_stream_until_request_message(&mut self) -> anyhow::Result<JSONRPCRequest> {
         loop {
             let message = self.read_jsonrpc_message().await?;
@@ -244,8 +240,6 @@ impl McpProcess {
         }
     }
 
-    // allow dead code
-    #[allow(dead_code)]
     pub async fn read_stream_until_response_message(
         &mut self,
         request_id: RequestId,
@@ -312,8 +306,6 @@ impl McpProcess {
         }
     }
 
-    // allow dead code
-    #[allow(dead_code)]
     pub async fn send_notification(
         &mut self,
         method: &str,