checker: fix generic type resolution in struct init with $if T is Type (fix #24471)

[kbkpbot]

…fix #24471)

When a generic method returns a generic struct initialized inside a $if T is Type branch, the compiler incorrectly resolved the function's generic parameter T using the struct init's concrete types instead of the function's concrete types.

Example that failed before:

  fn (thing GenericThing[T]) weird() AnotherGenericThing[int] {
      $if T is Type2 {
          return AnotherGenericThing[int]{thing.part1.item}  // error: int has no property item
      }
  }

The fix reorders the priority in unwrap_generic(): function-level generic parameters are now resolved before struct init context.