fix: improve type safety in httpWithTLS for custom RoundTripper (#861)

* fix: improve type safety in httpWithTLS for custom RoundTripper

Add type assertion check to handle cases where DefaultHTTPClient.Transport
is a custom http.RoundTripper implementation (not *http.Transport).

This improves upon the fix in PR #844 which only handled nil Transport.
The previous code would still panic if users set a custom RoundTripper:

  trans := baseTransport.(*http.Transport).Clone()  // panic if not *http.Transport

Now safely handles three scenarios:
1. Transport is nil -> use http.DefaultTransport
2. Transport is *http.Transport -> clone it
3. Transport is custom RoundTripper -> use http.DefaultTransport

Added comprehensive test cases:
- TestHttpWithTLS_NilTransport
- TestHttpWithTLS_CustomTransport
- TestHttpWithTLS_CustomRoundTripper

Related to #803

* refactor: reduce code duplication and complexity in httpWithTLS

- Eliminate duplicate http.DefaultTransport.Clone() calls
- Reduce cyclomatic complexity by simplifying conditional logic
- Use nil check pattern instead of nested else branches
- Maintain same functionality with cleaner code structure

This addresses golangci-lint warnings for dupl and gocyclo.

* fix: add newline at end of http_test.go

Fix gofmt -s compliance issue:
- File must end with newline character
- Addresses golangci-lint gofmt error on line 81

This fixes CI check failure.

util/http.go

@@ -292,13 +292,19 @@ func httpWithTLS(rootCa, key string) (*http.Client, error) {
 		Certificates: []tls.Certificate{cert},
 	}
 
-	var baseTransport http.RoundTripper
+	// 安全地获取 *http.Transport
+	var trans *http.Transport
+	// 尝试从 DefaultHTTPClient 获取 Transport，如果失败则使用默认值
 	if DefaultHTTPClient.Transport != nil {
-		baseTransport = DefaultHTTPClient.Transport
-	} else {
-		baseTransport = http.DefaultTransport
+		if t, ok := DefaultHTTPClient.Transport.(*http.Transport); ok {
+			trans = t.Clone()
+		}
 	}
-	trans := baseTransport.(*http.Transport).Clone()
+	// 如果无法获取有效的 Transport，使用默认值
+	if trans == nil {
+		trans = http.DefaultTransport.(*http.Transport).Clone()
+	}
+
 	trans.TLSClientConfig = config
 	trans.DisableCompression = true
 	client = &http.Client{Transport: trans}