feat(ws): add WithClientProxy option for explicit proxy configuration

[qiushido]

当前版本的 websocket 客户端在建立长链接时，底层默认使用了 ws.DefaultDialer 和 http.DefaultClient。这导致了在某些需要通过代理访问企业内网或外部服务的场景下，开发者无法为 websocket 客户端单独、显式地配置网络代理。

💡 方案 (Solution)
在 ws.Client 结构体内维护私有的 dialer 和 httpClient 实例，取代原先的全局单例，避免并发情况下的全局污染。
新增 WithClientProxy(proxyUrl string) 配置项：

• 解析传入的 HTTP 代理地址。
• 将代理应用到 ws.Client 专属的 Dialer 中。
• 深拷贝当前环境下的 http.DefaultTransport（若不存在则初始化基础 Transport），将代理应用其中，并构建专属的 http.Client 以用于获取建连 URL 时的 HTTP 请求。
  如果未显式调用 WithClientProxy，默认行为与之前保持一致（支持读取环境变量代理）。

💻 使用示例 (Example)

cli := larkws.NewClient(
    os.Getenv("APP_ID"), 
    os.Getenv("APP_SECRET"),
    larkws.WithEventHandler(eventHandler),
    // 显式配置 WebSocket 客户端的网络代理
    larkws.WithClientProxy("http://127.0.0.1:8888"),
)
err := cli.Start(context.Background())

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[Copilot]

Pull request overview

This PR adds an explicit proxy configuration option for the WebSocket client by moving away from global singletons (ws.DefaultDialer, http.DefaultClient) and introducing per-client networking primitives.

Changes:

• Add per-ws.Client instances of *websocket.Dialer and *http.Client to avoid global-side effects.
• Introduce WithClientProxy(proxyUrl string) to apply an explicit proxy to both the WebSocket dialer and the HTTP client used to fetch the connection URL.
• Route connection establishment and endpoint discovery through the client-owned dialer/http client.

---

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[Copilot]

url.Parse will succeed for many invalid proxy strings (e.g., missing scheme/host), and the resulting misconfiguration will only fail later at dial/request time. Consider validating that the parsed URL is absolute and has a supported scheme (http/https) and non-empty host, or using a stricter parser so invalid proxy inputs are rejected early.

Suggested change

	}
	}
	// Validate that the proxy URL is absolute, uses http/https, and has a host.
	if !proxy.IsAbs() || proxy.Host == "" {
	return
	}
	if proxy.Scheme != "http" && proxy.Scheme != "https" {
	return
	}

[Copilot]

When proxyUrl is non-empty but fails to parse, the option silently returns without applying any proxy. This makes misconfiguration hard to diagnose in production; consider surfacing this (e.g., store an error on the client to be returned from Start, or log a warning if a logger has already been provided).

Suggested change

	if err != nil {
	if err != nil {
	if cli.logger != nil {
	cli.logger.Warn(context.Background(), "invalid proxy URL %q: %v", proxyUrl, err)
	}

[Copilot]

The fallback transport = &http.Transport{} does not inherit the sensible defaults used by http.DefaultTransport (notably Dial/TLS handshake timeouts). If http.DefaultTransport has been replaced with a non-*http.Transport, this fallback can lead to connections without timeouts and unexpected hangs; consider initializing the transport with the same defaults as the standard library’s default transport instead of a zero-value Transport.

[Copilot]

WithClientProxy takes proxyUrl as a parameter name. For exported Go APIs, initialisms are typically capitalized (e.g., proxyURL) to match Go naming conventions and avoid inconsistent public surface naming.

Suggested change

	func WithClientProxy(proxyUrl string) ClientOption {
	return func(cli *Client) {
	if proxyUrl == "" {
	return
	}
	proxy, err := url.Parse(proxyUrl)
	func WithClientProxy(proxyURL string) ClientOption {
	return func(cli *Client) {
	if proxyURL == "" {
	return
	}
	proxy, err := url.Parse(proxyURL)