Update contextual validation of checkpointed merged ATXs

[fasmat]

Description

When the ATX handler receives a merged ATX in an epoch that was checkpointed additional rules for syntactic validation need to be added:

• If the merged ATX references a marriage ATX that a golden ATX also references it should be considered syntactically invalid and not stored in the nodes DB.
• Since the ATX is considered syntactically invalid, no malfeasance proof needs to be created for this case.

Implementation hints

See comments here:

go-spacemesh/activation/handler_v2.go

Lines 840 to 849 in 7b175da

	// TODO(mafa): during syntactical validation we should check if a merged ATX is targeting a checkpointed epoch
	// merged ATXs need to be checkpointed with their marriage ATXs
	// if there is a collision (i.e. the new ATX references the same marriage ATX as a golden ATX) it should be
	// considered syntactically invalid
	//
	// see https://github.com/spacemeshos/go-spacemesh/issues/6434
	otherAtx, err := h.fetchWireAtx(ctx, tx, other)
	if err != nil {
	return false, fmt.Errorf("fetching other ATX: %w", err)
	}