Stabilize download conformance tests under cache hits and fix chunk-cache nil race

[Copilot]

Recent conformance failures came from assuming both clients must always emit identical xorb download HTTP traffic. That assumption breaks when one side serves xorb data from local disk cache; additionally, a concurrent cleanup path could nil out disk cache state during prefetch persistence and panic.

• Conformance: cache-aware download assertions

  • Updated test/conformance/client/client_test.go to treat missing xorb HTTP requests as valid when caused by cache hits.
  • Kept strict request/range conformance checks when both clients do perform network xorb fetches.
  • Split logging paths for:
    • both clients served from cache
    • one client served from cache

• Runtime safety: guard prefetch cache writeback race

  • Updated download/chunk_cache.go to skip dc.put(...) when diskCache has been cleared by concurrent Done().
  • Added an inline comment documenting the cleanup/persistence race condition.

// Done() may run concurrently and clear diskCache before prefetch
// persistence reaches EOF; in that case we skip cache writeback.
if dc != nil {
	dc.put(hash, chunkStart, chunkEnd, bytesStart, bytesEnd, metas, file)
}