Use only lighthouse types in the mock builder (#4793)

## Proposed Changes

- only use LH types to avoid build issues
- use warp instead of axum for the server to avoid importing the dep

## Additional Info

- wondering if we can move the `execution_layer/test_utils` to its own crate and import it as a dev dependency
- this would be made easier by separating out our engine API types into their own crate so we can use them in the test crate
- or maybe we can look into using reth types for the engine api if they are in their own crate


Co-authored-by: realbigsean <seananderson33@gmail.com>

beacon_node/http_api/tests/tests.rs

@@ -265,11 +265,7 @@ impl ApiTester {
 
         // Start the mock builder service prior to building the chain out.
         harness.runtime.task_executor.spawn(
-            async move {
-                if let Err(e) = mock_builder_server.await {
-                    panic!("error in mock builder server: {e:?}");
-                }
-            },
+            async move { mock_builder_server.await },
             "mock_builder_server",
         );