fix(store): bind each gRPC stub to its own channel

[bitflicker64]

Purpose of the PR

• close [Bug][HStore Client] gRPC stub pools bind every entry to one ManagedChannel #3125

AbstractGrpcClient creates 32 managed channels per target, but both
stub-pool initialization loops bound every entry to one channel selected
before the loop. Traffic was concentrated on a single connection while the
other 31 channels remained idle.

Main Changes

• Bind each blocking-stub pool entry to the channel at the same pool index.
• Bind each asynchronous-stub pool entry to the channel at the same pool index.
• Add self-contained tests with fake channels and stubs that verify both pools
  cover every channel without a running PD or Store cluster.
• Add ClientSuiteTest so the existing store-client-test profile runs the
  new coverage without including cluster-dependent client tests.

Verifying these changes

• Need tests and can be verified as follows:
  • On the original indexing, both tests fail with 32 channels expected and 1 used.
  • mvn install -pl hugegraph-struct -am -DskipTests: passed.
  • mvn test -pl hugegraph-store/hg-store-test -am -P store-client-test: 2 passed, 0 failed.
  • mvn editorconfig:format: no files changed.
  • mvn clean compile -Dmaven.javadoc.skip=true: passed on Java 11.

Does this PR potentially affect the following parts?

• Dependencies
• Modify configurations
• The public API
• Other affects
• Nope

Documentation Status

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