Add Cross-DC L3VNI test cases: L3VNI_dci:51, 52, 61, 63

[bpar9]

Description of PR

Summary:
Implement 4 new Cross-DC L3VNI test cases from the DCI Solution Testplan, plus associated helper functions.

Test cases added to test_vxlan_dci.py:

• L3VNI_dci:51 (test_base_dci_l3vni_missing_vrf_on_dci_node): Negative test — removes VRF101 from one DC1 BGW, verifies Type-5 route withdrawal and traffic degradation, then restores VRF and verifies ECMP recovery across both BGWs.

• L3VNI_dci:52 (test_base_dci_l3vni_rt_mismatch): Negative test — modifies RT export on DC1 BGW to an incorrect value, verifies Type-5 routes are no longer imported on DC2 BGW and cross-DC traffic fails, then restores the correct RT and verifies traffic recovery.

• L3VNI_dci:61 (test_dci_l3vni_bulk_host_move): Host Mobility — learns 10 hosts simultaneously at dest1 (leaf0_dc1 orphan, VLAN 11, VRF 101), sends L2+L3 traffic from leaf2_dc1, moves all 10 hosts to dest2 (leaf0_dc2), verifies MM sequence numbers and L3VNI traffic, then moves back and verifies again.

• L3VNI_dci:63 (test_dci_l3vni_mh_to_mh_across_dc_with_l3_traffic): Host Mobility — MH-to-MH host move across DC with L3VNI inter-VLAN traffic verification via verify_mac_move_dci.

Note: L3VNI_dci:91 (VLAN remove/add on DC1-Leaf) is already fully implemented in test_vlan_remove_add with L3 traffic verification (line 5023 references L3VNI_dci:91).

Helper functions added to vxlan_helper.py:

• remove_vrf_config_bgw() — Remove VRF + L3VNI config from a BGW node
• restore_vrf_config_bgw() — Restore full L3VNI config on a BGW node
• modify_rt_export_bgw() — Set incorrect RT export for RT mismatch testing
• restore_rt_export_bgw() — Restore correct RT export
• generate_bulk_mac_move_hosts_dci() — Generate N host dicts (MAC/IPv4/IPv6) for bulk move tests

Type of change

• New Test case
  • Skipped for non-supported platforms

Approach

What is the motivation for this PR?

Extend Cross-DC L3VNI test automation coverage per the DCI Solution Testplan, covering negative scenarios (missing VRF, RT mismatch) and host mobility scenarios (bulk move, MH-to-MH with L3 traffic).

How did you do it?

• Added test methods to TestVxlanDCIBase (L3VNI_dci:51, 52) and TestVxlanDciMacMoveTriggers (L3VNI_dci:61, 63) classes following existing coding conventions (st.banner, st.log, vxlan_obj prefix, report_result pattern).
• Added reusable helper functions in vxlan_helper.py that leverage existing _get_l3vni_bgw_params() and generate_l3vni_bgw_*_config() infrastructure.
• L3VNI_dci:61 bulk move test creates 10 IXIA device groups, performs simultaneous move/move-back, and verifies MM sequence numbers + L3VNI traffic at each phase.
• L3VNI_dci:63 reuses verify_mac_move_dci() with mh_to_mh_across_dc direction and mac+ipv4 host type.

How did you verify/test it?

• Python compilation check passes (py_compile) for both modified files.
• Code follows existing test patterns and conventions in the codebase.

Supported testbed topology if it's a new test case?

3-datacenter EVPN-VXLAN DCI topology with IXIA traffic generation:

• DC1: 4 leafs + 2 spines + 2 BGW spines
• DC2: 2 leafs + 2 BGW spines
• DC3: 1 leaf + 1 BGW spine

Link to Devin session: https://cisco-demo.devinenterprise.com/sessions/173ce16b4ed5458ba1a5722abb95d2f7
Requested by: @bpar9

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