chore: replace testing.Testing with flag lookup (#26552)

In our codebase we have an existing convention of using
`flag.Lookup("test.v")` instead of `testing.Testing()`. This avoids
pulling in the entire `testing` package. Another consequence: some of
our custom linters trigger upon import of the `testing` package which
can lead to unexpected linter errors.

agent/x/agentmcp/mcphttpclient.go

@@ -1,8 +1,8 @@
 package agentmcp
 
 import (
+	"flag"
 	"net/http"
-	"testing"
 )
 
 // mcpHTTPClient returns an isolated *http.Client when running
@@ -13,7 +13,7 @@ import (
 // is a *http.Transport it is cloned; otherwise a minimal
 // transport with ProxyFromEnvironment is created as a fallback.
 func mcpHTTPClient() *http.Client {
-	if !testing.Testing() {
+	if flag.Lookup("test.v") == nil {
 		return nil
 	}
 	if dt, ok := http.DefaultTransport.(*http.Transport); ok {