fix: robustify form helper types

[dummdidumm]

Optional properties make the resulting type X | undefined which when turned into an intersection results in the never type. We therefore mark all properties as required while transforming the objects into their desired shape.

There's another adjacent bug: If you have an object only consisting of optional properties, e.g. {a?: string}, then RemoteFormInput extends it and the type is degraded to e.g. { [key: string]: RemoteFormIssues[] }. A fix to the "will recurse indefinetly" helper type solves this: we now check if the object keys match an index signature, and only bail in that case.

Also added a few tests

Fixes #14461

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[ottomated]

Duplicate of #14458? I think my approach is less likely to create indefinite type nesting errors

[dummdidumm]

Sorry for not seing that PR earlier - your solution looks similar with respect to preventing undefined from sneaking in. I chose to go with Required<T> instead (feels a bit "cleaner", purely aesthetic preference), and fixes and adjacent bug around false-positive "this is infinite"