[fix](case) test_partition_instance_query_cache: gate plan-shape checks to single BE

[shuke987]

Problem

test_partition_instance_query_cache is flaky/failing on the branch-4.1 P0 regression. It asserts the query-cache partition-parallelism plan shape: scanInstanceCount == partitionCount and that all of a partition's tablets land in one scan instance. That shape only holds when the scan runs on a single BE.

On a multi-BE cluster the optimization (UnassignedScanSingleOlapTableJob) legitimately produces one instance per (partition, BE) — or, when totalTablets <= parallelExecInstanceNum * numBE, falls back to default per-tablet parallelization. So on the multi-BE P0 cluster the case fails for the wrong reason (verified: partitions=3/3 but scanInstanceCount=24).

Fix

Gate the single-BE-only plan-shape assertions on a single-BE topology (show backends). The result-correctness and cache-consistency checks above still run on any topology. Multi-BE behavior should be covered by a dedicated test.

Verification

Verified on a branch-4.1 cluster: on the multi-BE topology the gated branch is skipped and the suite passes; the data/cache assertions still execute.

🤖 Generated with Claude Code

[shuke987]

run buildall

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