docs(hugegraph): mark distributed systemAccounts unsupported

[weicao]

Summary

Child PR into dev/hugegraph-release-1.0. Close the 12a systemAccounts row for topology=distributed.

Distributed PD / Store / Server do not declare systemAccounts and do not generate an admin Secret. BackupPolicyTemplate stays on the standalone server pattern only.

Contract

• docs/addon-api/12a-minimum-acceptance.md: each declared topology needs systemAccounts closed or a restriction statement.
• docs/addon-api/12b-claimed-only-acceptance.md: unclaimed capabilities must not be written as supported.

Test

bash addons/hugegraph/tests/contract_test.sh
HugeGraph addon offline contracts passed

Main PR: #3412

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (754d5d6) to head (a3ff7fe).
⚠️ Report is 1 commits behind head on dev/hugegraph-release-1.0.

Additional details and impacted files

@@                    Coverage Diff                    @@
##           dev/hugegraph-release-1.0   #3416   +/-   ##
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  Coverage                       0.00%   0.00%           
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  Files                             69      69           
  Lines                           7275    7275           
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  Misses                          7275    7275           

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