[reconfigurator] Improve blocked MGS driven update code

There are several pending items in regards to improving the style and readability for the blocked MGS driven updates code.

For context:
https://github.com/oxidecomputer/omicron/pull/9001#discussion_r2372837627
https://github.com/oxidecomputer/omicron/pull/9001#discussion_r2372842378
https://github.com/oxidecomputer/omicron/pull/9001#discussion_r2372863166

Follow up to #9001 