fix(querytee): Fix TestProxyEndpoint_QuerySplitting clock-drift bug

[jnewbigin]

Summary

• TestProxyEndpoint_QuerySplitting was flaking in CI with 0/3 instead of 2/1 for the old/recent query split classification.
• Root cause: the test captures threshold = time.Now().Truncate(time.Minute) - 3h once at setup, but the engine router computes v2End = time.Now() - StorageLag per request. For logs queries, alignV2Range is a no-op (step=0), so v2End drifts second-by-second with wall-clock. When the truncation landed late in a minute and the prior 2-second sleep pushed total drift past 60s, v2End > threshold + 60s and all three queries got misclassified as "recent".
• Fix is test-only: classify queries by whether their end matches the request's end (the post-v2 split runs to request end; the v2 split ends earlier) instead of an absolute boundary, and widen the mock server's split boundary from threshold + 60s to threshold + 30m so it stays stable under any plausible drift.

Test plan

• go test -tags=assert -count=10 -run TestProxyEndpoint_QuerySplitting ./pkg/querytee/... passes (10/10)
• go test -tags=assert ./pkg/querytee/... passes

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Note

Low Risk
Low risk because changes are confined to test logic and only adjust how the mock split boundary and assertions are computed to avoid time.Now() drift flakes.

Overview
Fixes CI flakiness in TestProxyEndpoint_QuerySplitting by making the mock backend’s split boundary drift-tolerant (using threshold + 30m and treating equality as recent).

Updates assertions to classify old vs recent sub-queries based on whether the query end matches the original request end (instead of comparing against a step-aligned absolute boundary), and removes the now-unused stepAlignUp helper.

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