Resolve outstanding comments on a past MR to the ready_to_merge widget

What does this MR do and why?

For #479936 (closed)

Mostly, this just cleans up some tests and tightens up the spec file.

However, it also removes a v-model/@input pair to avoid the duplicate value handling by replacing it with :value/@input.

References

• Definitely please see the issue, which came from comments on another MR

MR acceptance checklist

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Screenshots or screen recordings

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