Missing test coverage: Retry mutations

While working on #353293 (closed), we found that a refactor inadvertently caused errors when mutations were retried. Only one test caught this error, test_large_har_report_with_assets in worker-entry; and it was only found accidentally, because of the order of the operations in the har file. Fortunately the error was severe and immediate, so the bug was easily found from the stack trace despite the test being very high level. But there seems to be a need for more specific testing around retrying mutations.

/cc @mikeeddington