normalizeDollarType

Normalize a $type discriminator value for equality matching.

Per the AT Protocol data model spec, a $type of com.example.foo and com.example.foo#main refer to the same definition — the #main fragment is implicit and MAY be omitted. Generators always match against the bare form, so incoming values with a #main suffix are stripped.

Other fragments (e.g. com.example.foo#bar) are preserved as-is.