[worker-versioning]: blackhole queries only on root partition

[Shivs11]

What changed?

• We should be checking if a query is blackholed only by checking the appropriate conditions on the root partition.
• Moreover, added the testhook bug about read partitions over here which @dnr spotted!

Why?

• Correctness.
• Moreover, the test TestVersioning3FunctionalSuiteV0/TestPinnedQuery_RollbackDrainedVersion was failing after the testhook bug fix. The root partition check also fixed it!

How did you test it?

• built
• run locally and tested manually
• covered by existing tests
• added new unit test(s)
• added new functional test(s)

Potential risks

• None.

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Note

Ensures query blackhole detection only runs on the root partition to avoid false "no pollers" errors across partitions.

• Update checkQueryBlackholed to early-return on non-root partitions and add clarifying comments
• Fix load balancer testhook to use MatchingLBForceReadPartition when forcing read partition selection

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