fix: graceful shutdown wg tracking for issues #245, #221, #220

[poyrazK]

Summary

• Graceful shutdown missing wg.Wait() - goroutines may not complete #245 (Critical): Add s.wg.Wait() to defer after close(s.done) so Run() blocks until all goroutines exit. Close listeners in the return path before wg.Wait() to unblock accept loops.
• NOTIFY handler goroutine escapes wg tracking and uses request-scoped context #221 (Critical): Track NOTIFY handler goroutine with s.wg.Add(1)/defer s.wg.Done() so it is counted in the wait.
• Goroutine leak - dlqRetryWorker ignores s.done shutdown signal #220 (Critical): Add case <-s.done: to dlqRetryWorker select so it exits on server shutdown.
• Bonus: Close DoQ listener before wg.Wait() (was already closed in defer but needed before wait).
• cache.go: Remove broken wg tracking from NewDNSCache (call was racing with goroutine start).
• doq.go: Add ctx.Done() check to DoQ listener accept loop.
• Tests: Updated TestServer_Run_ContextCancel and TestServer_Run_NonBlockingOnTCPDoTFailure timeouts from 500ms to 3s to accommodate proper wg.Wait() behavior.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 2478afbe-1edb-496b-8a29-7cd8239a8e40

📥 Commits

Reviewing files that changed from the base of the PR and between 5908d38 and 92248cd.

📒 Files selected for processing (5)

• internal/dns/server/cache.go
• internal/dns/server/cache_test.go
• internal/dns/server/doq.go
• internal/dns/server/server.go
• internal/dns/server/server_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/graceful-shutdown-wg-fix

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