Call resetFormState if the board comes through after the modal

What does this MR do and why?

Call resetFormState in issue boards if the board comes through after the modal.

Currently, resetFormState is called on mount and never again. If you open the board configuration modal before the board query has resolved, the form never initialises properly and the "Save" button never gets enabled, because the name never gets filled out!

This causes flakes in tests:

• https://gitlab.com/gitlab-org/gitlab/-/jobs/15822258321 — 2026-08-11, first noted occurrence, master pipeline
• https://gitlab.com/gitlab-org/gitlab/-/jobs/16056289548 — 2026-08-23, master pipeline
• https://gitlab.com/gitlab-org/gitlab/-/jobs/16197398772 — 2026-08-30, MR pipeline

We also rewrite some related specs to not rely on wait_for_requests, as part of Remove all uses of `wait_for_requests`, `wait_f... (&22879), which is what led us to this emerging bug.

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