chore: import upstream snapshot with attribution

internal/hook/testing.go

@@ -0,0 +1,23 @@
+// Copyright (c) 2026 Lark Technologies Pte. Ltd.
+// SPDX-License-Identifier: MIT
+
+package hook
+
+import "io"
+
+// SetStderrForTesting redirects the hook layer's warning output to a
+// custom writer and returns a restore function the caller MUST defer
+// (or pass to `t.Cleanup`). Without the restore step, a later test in
+// the same binary would inherit the override and either race on a
+// shared bytes.Buffer or write user-visible garbage into a real test
+// stderr.
+//
+// Production code never calls this; the default writer is os.Stderr
+// via defaultStderr.
+func SetStderrForTesting(w io.Writer) (restore func()) {
+	prev := stderr
+	stderr = func() interface{ Write(p []byte) (int, error) } {
+		return w
+	}
+	return func() { stderr = prev }
+}