Fix VLAN member tagging_mode updates for existing members

[johanna-nexthop]

What I did

This change fixes VLAN member tagging_mode updates for ports that are already members of a VLAN.

Previously, duplicate VLAN member entries were treated as no-ops in both vlanmgrd and PortsOrch, which prevented valid tagging_mode updates from being applied to existing members. This PR changes that behavior so an existing VLAN member is only skipped when the requested tagging_mode already matches the applied state.

The update includes:

• vlanmgrd now stores and checks tagging_mode in STATE_DB, so it can distinguish a true duplicate from a legitimate mode change, including after restart.
• PortsOrch now handles existing VLAN members by applying a tagging_mode update instead of always treating them as duplicates.
• A helper was added to convert string tagging modes to SAI enums and reuse that logic consistently.
• Because the vendor SAI path does not support in-place updates of SAI_VLAN_MEMBER_ATTR_VLAN_TAGGING_MODE, the implementation falls back to remove-and-recreate of the VLAN member object while reusing the bridge port.
• Tests were added and updated across DVS and mock/unit coverage, including new vlanmgrd unit test wiring in Makefile.am.

Why I did it

Updating the tagging_mode of an existing VLAN member should be handled as a real configuration change, not ignored as a duplicate.

Without this fix:

• valid tagging_mode updates can be dropped because the existing member is treated as already programmed
• restart and replay behavior can lose track of the applied mode
• host-side VLAN programming can fail during repeated default VLAN cleanup when replacing an existing member
  This change ensures VLAN member mode updates are applied correctly, tracked persistently, and handled safely during replay and replacement.

How I verified it

• Added and updated DVS test helpers to support VLAN member tagging_mode updates.
• Updated verification logic to re-resolve the ASIC VLAN member object during polling, since the object is replaced when the tagging mode changes.
• Added vlanmgrd mock/unit test coverage and wired it into tests/mock_tests/Makefile.am.
• Included coverage in both mock/unit tests and VLAN DVS tests.

Details if related

[mssonicbld]

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[mssonicbld]

Hi, there are workflow run(s) waiting for approval, you may be first-time contributor. I will notify maintainers to help approve once PR is approved. Thanks!

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