chore(ci): enable hugegraph-struct tests

[contrueCT]

Purpose of the PR

• close [Feature] enable hugegraph-struct tests in CI #3018

Enable CI coverage for hugegraph-struct tests in the PD/Store workflow.

Currently, hugegraph-struct is installed as an upstream dependency with -DskipTests, so regressions covered only by struct-side tests may not be caught by PR CI.

Main Changes

• Keep the existing build/fetch behavior for hugegraph-struct
• Add a dedicated Run hugegraph-struct test step when the module source exists
• Continue to let PD/Store/HStore jobs depend on the struct job as before

Verifying these changes

• Trivial rework / code cleanup without any test coverage. (No Need)
• Already covered by existing tests, such as (please modify tests here).
• Need tests and can be verified as follows:
  • mvn -U -ntp "-Deditorconfig.skip=true" -pl hugegraph-struct test

Does this PR potentially affect the following parts?

• Dependencies (add/update license info & regenerate_known_dependencies.sh)
• Modify configurations
• The public API
• Other affects (typed here)
• Nope

Documentation Status

• Doc - TODO
• Doc - Done
• Doc - No Need