Implement strict VLAN validation in sdx-lib to prevent invalid L2VPNs

[gretelliz]

What this PR does

• Adds validation in sdxlib/validator.py for VLAN values, addressing the issues described in Reject unsupported VLAN keyword “all” client-side and normalize “any” / “untagged” with consistent endpoint checks sdx-controller#496

• Rejects unsupported "all" keyword

• Enforces consistency when using special keywords ("any" or "untagged")

• Prevents mixing special keywords with explicit VLAN IDs

• Validates numeric VLANs are within the valid range (1–4094)

• Integrated the validation into create_l2vpn()

• Added unit tests covering all critical cases:

  • explicit same VLAN → success
  • any + any → success
  • untagged + untagged → success
  • any + explicit → mismatch error
  • untagged + any → mismatch error
  • all + all → unsupported error
  • all + explicit → unsupported error

Related issue

Closes atlanticwave-sdx/sdx-controller#496

Testing

• All existing tests pass
• New parametrized tests added in tests/test_validator.py specifically verify the 7 required cases
• Coverage increased in validator.py and l2vpn.py

To run only the new tests (ignoring the old ones in old_tests/):

pytest --cov=sdxlib --cov-report=term-missing -v tests/