[mohankarthik]

Summary

Follow-up to #750 fixing hybrid and multi-VLAN port handling, RemoveVlanMember cleanup, and vlan map thread safety on main:

1. Synchronize vlan Map Access (vlan.mu):
   • Protect all reads and writes to vlan.members and vlan.vlans (vidByOid, oidByVid, memberByOid, memberByPortId, untaggedMemberByPortId, hasTaggedMember, CreateVlan, CreateVlanMember, RemoveVlanMember, Reset) with vlan.mu (sync.RWMutex).
2. Preserve Existing VLAN Memberships on Hybrid & Multi-VLAN Trunk Ports (untaggedMemberByPortId):
   • Previously, CreateVlanMember called memberByPortId(portID) and deleted any existing VLAN membership on the port before adding the new one.
   • Replace memberByPortId eviction with untaggedMemberByPortId(portID) and only evict when the new membership is UNTAGGED / PRIORITY_TAGGED, allowing multiple TAGGED VLANs (trunk port) and one UNTAGGED native VLAN + TAGGED VLANs (hybrid port) to coexist regardless of creation order.
3. Update RemoveVlanMember Lifecycle:
   • Remove the deleted member OID from the parent VLAN's MemberList attribute in attrmgr.
   • When memberships remain on the port after removal, reprogram VlanTable according to the remaining memberships (restoring untagged Encap(ETHERNET_VLAN) + Update(VLAN_TAG) actions when the last TAGGED membership is removed from a hybrid port); when no memberships remain on the port, remove its VlanTable entry via TableEntryRemove.
4. Unit Tests (vlan_test.go):
   • Fix && -> || assertion bug in TestVlanOperations.
   • Add 5 subtests in TestCreateVlanMemberHybridPort covering UNTAGGED->TAGGED, TAGGED->UNTAGGED, multi-VLAN TAGGED trunk ports, untagged VLAN replacement on a hybrid port, and tagged/untagged member removal.