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feat: implement upstream connection pooling (#127)

* feat: implement upstream connection pooling

- Introduce `ConnPool` to reuse DNS upstream connections.
- Add `--connection-pool-size` flag (default 10) to configure the pool.
- Add `dns_pool_evictions_total` metric to track pool capacity issues.
- Refactor `RoundRobinClient` to manage pools per upstream.

* fix: prevent panic on multiple DNSCache Close calls

- Protect `done` channel closure with `sync.Once`.
- Switch local test DNS server to TCP to prevent race conditions during
port allocation.
- Use `ConnPool` for health checks instead of re-instantiating
`dns.Client` in `RoundRobinClient`.

* feat: implement connection idle timeout in ConnPool

- Add `maxIdleAge` to `ConnPool` to expire idle connections after 8s.
- Refactor `acquire` and `release` logic to encapsulate connection
  lifecycle management and stale connection cleanup.
- Remove redundant upstream entries in `main.go`.

* feat: add validation for connection pool size

Ensure the connection pool size is a non-negative integer to prevent
invalid configurations during client initialization.

* refactor: remove redundant sync.Once and fix test leaks

- Removed `sync.Once` from `DNSCache.Close()` as it is unnecessary for
  closing a single-use channel.
- Moved `cache`, `metrics`, and `dnsClient` initialization inside the
  test cases in `TestDNSDispatcher_QueryLogging` to ensure each test
  instance uses a fresh cache and prevents cross-test interference.

* test: fix race condition in local DNS test server

Updated `startLocalDNS` to pass an already-bound listener to the DNS
server. This prevents race conditions where the port might be grabbed
by another process between `net.Listen` and `ListenAndServe`.

* Update internal/forwarder/round_robin_client.go

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

* fix: retry once on reused connection failure

If a connection acquired from the p... (continued)

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