[slack-22.0] backport: fix: consolidator waiter cap fallback to independent execution (#716)

[tanjinx]

• fix: consolidator waiter cap fallback to independent execution

When the consolidator waiter cap is reached, queries should fall back to independent execution instead of returning empty results.

Before this fix:

• Queries exceeding waiter cap would skip waiting for consolidation
• They would immediately try to access q.Result() before completion
• This resulted in empty/incomplete results being returned

After this fix:

• Queries exceeding waiter cap fall back to regular execution path
• All queries return correct results regardless of consolidation status
• Waiter cap configuration still controls resource usage as intended

Changes:

• Modified execSelect() in query_executor.go to implement fallback logic
• Enhanced FakePendingResult to properly simulate waiter count behavior
• Added comprehensive test TestQueryExecutorConsolidatorWaiterCapFallback

• implement consolidator-query-waiter-cap-method

• fix help message

• fix redundant code

• fix test

• feat: add consolidator-query-waiter-cap-method config parameter

Add --consolidator-query-waiter-cap-method flag to control behavior when consolidator waiter cap is exceeded. Options:

• 'fallthrough' (default): Fall back to independent query execution
• 'reject': Return RESOURCE_EXHAUSTED error

This provides operators fine-grained control over consolidator memory management while maintaining backward compatibility.

Changes:

• Add ConsolidatorQueryWaiterCapMethod config field and CLI flag
• Update execSelect() to handle both reject and fallthrough behaviors
• Add comprehensive test coverage for both methods
• Add config validation with graceful defaults
• Fix waiter counter cleanup to ensure proper resource management

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• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

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