fix: [proxy/engines] dpc chunk test wallclock dependency

[crandles]

Description

Three chunk tests in pkg/proxy/engines/deltaproxycache_chunk_test.go hardcode now := time.Date(2026, 4, ...) and fail once real wallclock moves past step * TimeseriesRetention (1h * 1024 = ~42.67d) beyond the fixture date. TestDeltaProxyCacheRequestRangeMissChunks started failing locally around 2026-05-25, the two _CrossBucket siblings would have followed a few days later.

The trigger is the EvictionMethodOldest branch in deltaproxycache.go:238-251 -- OldestRetainedTimestamp is computed from time.Now(), so a fixed-calendar trq.Extent.End eventually slides before it and DPC short-circuits to DoProxy (engine=HTTPProxy, status=proxy-only). The fixture comment about the "17.5d chunk bucket" was about a different thing (chunk size = step * DefaultTimeseriesChunkFactor); it isn't what was tripping the test.

First commit is intentionally empty so CI captures the failure on a clean branch off main, leaving a permanent record of the time bomb. Second commit replaces the hardcoded dates with two helpers anchored to time.Now():

• chunkBucketAnchor(step) for the single-bucket case (TestDeltaProxyCacheRequestRangeMissChunks) -- midpoint of a recently-completed chunk bucket
• chunkBucketStraddle(step) for the cross-bucket cases -- 11h after a recent chunk-bucket boundary so Q1 (-30h..-12h) and Q2 (-10h..-8h) fall on opposite sides

Both helpers stay well inside the retention window so future wallclock drift can't reintroduce the same failure.

Type of Change

• • Bug fix

AI Disclosure

• • This contribution DOES include AI-generated changes, and I have reviewed the relevant contributing guidelines.

[crandles]

merged to #1012