fix(hugegraph): derive PD initial store count from store FQDNs

[weicao]

Summary

Child PR into dev/hugegraph-release-1.0.

Official HugeGraph PD requires pd.initial-store-count to match the expected Store count (3 for a 3-store cluster). start-pd.sh already builds HG_PD_INITIAL_STORE_LIST from STORE_POD_FQDNS but hardcoded HG_PD_INITIAL_STORE_COUNT=1. That can mark the cluster operational after the first Store and leave later Stores out of the first partition allocation.

Derive the default count from the Store FQDN list. An explicit HG_PD_INITIAL_STORE_COUNT still wins.

Test

bash addons/hugegraph/tests/start_pd_test.sh
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 (bb20a02) to head (a26bdfb).
⚠️ Report is 1 commits behind head on dev/hugegraph-release-1.0.

Additional details and impacted files

@@                    Coverage Diff                    @@
##           dev/hugegraph-release-1.0   #3419   +/-   ##
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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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