test(proxy): fix data race and flaky deadline test in v2 CI

Two pre-existing issues in middleware/proxy made v2 CI red under
`go test -race` on newer Go versions and Windows:

1. Data race: Test_Proxy_Forward_WithTlsConfig called WithTlsConfig(),
   which mutates the field client.TLSConfig on the package-global client
   with no synchronization against fasthttp's concurrent read of that
   field from parallel proxy tests. Swap the whole global client via
   WithClient(&fasthttp.Client{TLSConfig: ...}) instead (pointer swap
   under the existing lock), mirroring the v3 fix. The public
   WithTlsConfig API is left untouched.

2. Flaky timing: Test_Proxy_DoDeadline_PastDeadline used a 1s DoDeadline
   deadline equal to app.Test's default 1000ms timeout, so two coincident
   timers raced. Bump the deadline to 2s so app.Test's timeout fires
   first deterministically (same approach as v3).

Test-only change, no production or API changes. Verified 12/12 green
with `-race -shuffle=on`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ReneWerner87

middleware/proxy/proxy_test.go

@@ -249,8 +249,9 @@ func Test_Proxy_Forward_WithTlsConfig(t *testing.T) {
 	addr := ln.Addr().String()
 	clientTLSConf := &tls.Config{InsecureSkipVerify: true} //nolint:gosec // We're in a test func, so this is fine
 
-	// disable certificate verification
-	WithTlsConfig(clientTLSConf)
+	// disable certificate verification by swapping the whole global client
+	// (WithTlsConfig mutates the shared client's field and races with parallel proxy tests)
+	WithClient(&fasthttp.Client{TLSConfig: clientTLSConf})
 	app.Use(Forward("https://" + addr + "/tlsfwd"))
 
 	go func() { utils.AssertEqual(t, nil, app.Listener(ln)) }()
@@ -565,7 +566,9 @@ func Test_Proxy_DoDeadline_PastDeadline(t *testing.T) {
 
 	app := fiber.New()
 	app.Get("/test", func(c *fiber.Ctx) error {
-		return DoDeadline(c, "http://"+addr, time.Now().Add(time.Second))
+		// Deadline is longer than app.Test's 1000ms timeout so the test timeout
+		// deterministically fires first (avoids two coincident 1s timers racing).
+		return DoDeadline(c, "http://"+addr, time.Now().Add(2*time.Second))
 	})
 
 	_, err1 := app.Test(httptest.NewRequest(fiber.MethodGet, "/test", nil))