test: guard synchronous DPC cache writes

[houyuwushang]

Description

#930 made DPC cache writes synchronous inside the singleflight executor, removing the background access reported in #765. The chunked range-miss test still slept for 10 ms to wait for that old goroutine.

Remove the delay and issue the next request immediately. The existing kmiss to rmiss assertions now guard that the first handler does not return before its cache write completes. As a mutation check, moving WriteCache back into a goroutine made the test fail with status=kmiss.

Closes #765.

Tests:

• go test ./pkg/proxy/engines -count=1
• go test -race ./pkg/proxy/engines -count=1
• go test -race ./pkg/proxy/engines -run '^TestDeltaProxyCacheRequestRangeMissChunks$' -count=1000
• go tool golangci-lint run -c .golangci.yml ./pkg/proxy/engines/...

Type of Change

• • Bug fix
• • New feature
• • Optimization
• • Test coverage
• • Documentation
• • Infrastructure

AI Disclosure

• • This contribution DOES NOT include AI-generated changes
• • This contribution DOES include AI-generated changes, and I have reviewed the relevant contributing guidelines.