fix: Call hooks from unexported embedded fields

[abhinav]

Bug:
In #616, Kong stopped calling compiler-generated promoted methods.
It traversed exported embedded fields instead so each hook runs at its
declaring receiver.

That traversal cannot enter an unexported anonymous field. The promoted
method was ignored and the declaring receiver was skipped, so the hook
did not run. For example:

type options struct{}

func (*options) AfterApply() error { return nil }

type CLI struct {
    options
}

CLI.AfterApply is callable through Go's promoted method wrapper,
but the callback search introduced by #616 rejected that wrapper
and could not visit options through reflection.

Fix:
Fall back to the promoted method when traversal finds no explicit hook.
Exported embedded hooks still run at their declaring receiver once.
An unexported embedded hook runs through the callable parent wrapper.

Validation:
Includes a regression test that fails without this fix.

[abhinav]

This change is part of the following stack:

• fix: Call hooks from unexported embedded fields #620 ◀

_{Change managed by git-spice.}