HBASE-30099 Avoid NPE when GetBootstrapNodes RPC arrives during startup

[Sigma-Ma]

What changes were proposed in this pull request?

Make HRegionServer#getBootstrapNodes return an empty iterator when bootstrapNodeManager has not been initialized yet.

Declare bootstrapNodeManager as volatile to ensure safe publication between the RegionServer startup thread and RPC handler threads.

Add a regression test covering calls to getBootstrapNodes before bootstrapNodeManager initialization.

Why are the changes needed?

The RPC server can start accepting connections before preRegistrationInitialization() assigns bootstrapNodeManager.

If a GetBootstrapNodes RPC arrives during this startup window, HRegionServer#getBootstrapNodes dereferences a null bootstrapNodeManager and throws a NullPointerException.

Returning an empty iterator during this window avoids the startup race and allows callers to continue bootstrap-node discovery normally.

How was this patch tested?

mvn -pl hbase-server -am \
  -Dtest=TestBootstrapNodeManager \
  -Dsurefire.failIfNoSpecifiedTests=false \
  -DskipITs \
  test

[Sigma-Ma]

Hi maintainers, the three GitHub Actions workflows for this fork PR are awaiting approval. This PR does not modify any workflow files.

Could someone with write access approve them when convenient? Thanks.