[SDTEST-2702] Add local parallelism for CI nodes

[anmarchenko]

Summary

• Add --ci-node-workers setting (default: CPU count) to control how many parallel workers run within each CI node
• When running in CI-node mode with multiple workers, tests assigned to the node are split among local workers and executed in parallel via errgroup
• {{nodeIndex}} in worker-env is now a global worker index calculated as ciNode * ciNodeWorkers + localWorkerIndex, ensuring unique indices across all workers on all nodes
• Fix edge case in parallelism calculation when maxParallelism is 0 or negative

Test plan

• Added tests for splitTestFilesIntoGroups() helper (even/uneven splits, edge cases)
• Added tests verifying global index calculation (e.g., ci-node=1 with 2 workers yields indices 2 and 3)
• Added tests for single-worker and multi-worker CI-node modes
• Added tests for ci_node_workers setting defaults and env/flag overrides
• Added tests for maxParallelism <= 0 edge case
• make test and make lint pass

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Ensure global worker indices stay unique across CI nodes

The global index formula here (ciNode*ciNodeWorkers + localIndex) assumes every CI node uses the same ciNodeWorkers value, but runCINodeTests sources it from settings (default runtime.NumCPU) per node. In heterogeneous CI pools (different CPU counts), this yields overlapping indices (e.g., node0 with 8 workers produces 0–7, node1 with 4 workers produces 4–7), which defeats the stated uniqueness guarantee and can cause {{nodeIndex}}-based worker envs (DB names, ports) to collide across nodes unless users manually align ci-node-workers everywhere.

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