fix: auth typings and reduced type ambiguity

[damusix]

• RFC, claude code helped me with this one. It looks correct.
• Fixed regression in MergeType utility that broke auth Possible typing regression between v21.3.12 and v21.4.4 #4561
• Add type missed merges
• Remove ambiguous types
• Added more TS tests. LMK if you want me to remove code comments

[fjeldstad]

This should have been part of a major version bump as the type changes are breaking in some scenarios. For example when extracting a header value; that used to be string | undefined while now it is string | string[] | undefined meaning that it cannot be assigned to a string variable anymore. Even if it is more correct now, I don't think consumers expect the build to break from installing a patch version bump.

[Marsup]

@fjeldstad we already answered that in #4563. Breaking types is inherent to the typesrcipt ecosystem, nobody releases a major version just for types correctness.

[kanongil]

nobody releases a major version just for types correctness

FYI, some projects do try to do that by eg. adhering to the proposed semver-ts spec. However, even with that, changes in minor releases can cause type compilation issues! As they state (paraphrased):

We propose the rules below, with the caveat that they will not prevent all possible breakage—only the majority of it.

As such, for typescript development, one must expect that any typings update can potentially cause "breakage".