fix: account for current guest nodes and allow anti-affinity migrations from non-peak nodes

[dguerri]

Description

1. Calculations.val_anti_affinity used used_nodes which starts empty and only recorded nodes chosen during the current relocation run. It did not check the existing locations of other members in the anti-affinity group, allowing guests to be assigned to nodes already hosting another member of the group.
2. Calculations.relocate_guests unconditionally broke out of the guest relocation loop if the guest's source node was not the highest-loaded node in the cluster (highest_node.name != source_node). If anti-affinity group members were co-located on a non-peak node, ProxLB would never move them to resolve the violation.

Solution

• Pre-populate occupied_nodes in val_anti_affinity with the current or target nodes of all other members of the anti-affinity group before choosing an assignment.
• Exempt anti-affinity-violating guests from the highest_node != source_node early-exit when enforce_affinity is active.

Added unit tests in tests/test_calculations_anti_affinity_relocation.py validating anti-affinity target selection and non-peak node relocation.