[copp] test_copp.py missing BGPv6 test coverage

[nhegde-microsoft]

Description

tests/copp/test_copp.py (sonic-mgmt#27726) does not have any BGPv6 test coverage.

PROTOCOL_TO_TRAP_ID lists "BGP": ["bgp", "bgpv6"], but every BGP-related test call (test_add_new_trap, test_remove_trap, test_policer_mtu) hardcodes ip_version="4". No test has ever sent IPv6 BGP traffic for this trap.

Acceptance Criteria

• Add BGPv6 test coverage to tests/copp/test_copp.py for the existing BGP-related test cases (test_add_new_trap, test_remove_trap, test_policer_mtu), either by parametrizing ip_version to include "6" for the BGP trap or adding equivalent BGPv6-specific cases.