refactor(agent): extract agent pool and HTTP utilities to dedicated package

Moved non-agent-specific logic from agent/pkg/agent/ to internal/agentpool/:
- pool.go: Agent pool management (Get, Add, Remove, List, Iter, etc.)
- http_requests.go: HTTP utilities (health checks, forwarding, websockets, reverse proxy)
- agent.go: Agent struct with HTTP client management

This separates general-purpose pool management from agent-specific configuration,
improving code organization and making the agent package focused on agent config only.

internal/agentpool/agent.go

@@ -0,0 +1,54 @@
+package agentpool
+
+import (
+	"net"
+	"net/http"
+	"time"
+
+	"github.com/valyala/fasthttp"
+	"github.com/yusing/godoxy/agent/pkg/agent"
+)
+
+type Agent struct {
+	*agent.AgentConfig
+
+	httpClient       *http.Client
+	fasthttpHcClient *fasthttp.Client
+}
+
+func newAgent(cfg *agent.AgentConfig) *Agent {
+	transport := cfg.Transport()
+	transport.MaxIdleConns = 100
+	transport.MaxIdleConnsPerHost = 100
+	transport.ReadBufferSize = 16384
+	transport.WriteBufferSize = 16384
+
+	return &Agent{
+		AgentConfig: cfg,
+		httpClient: &http.Client{
+			Transport: transport,
+		},
+		fasthttpHcClient: &fasthttp.Client{
+			DialTimeout: func(addr string, timeout time.Duration) (net.Conn, error) {
+				if addr != agent.AgentHost+":443" {
+					return nil, &net.AddrError{Err: "invalid address", Addr: addr}
+				}
+				return net.DialTimeout("tcp", cfg.Addr, timeout)
+			},
+			TLSConfig:                     cfg.TLSConfig(),
+			ReadTimeout:                   5 * time.Second,
+			WriteTimeout:                  3 * time.Second,
+			DisableHeaderNamesNormalizing: true,
+			DisablePathNormalizing:        true,
+			NoDefaultUserAgentHeader:      true,
+			ReadBufferSize:                1024,
+			WriteBufferSize:               1024,
+		},
+	}
+}
+
+func (agent *Agent) HTTPClient() *http.Client {
+	return &http.Client{
+		Transport: agent.Transport(),
+	}
+}