all: add a test that checks for missing or mismatching file copyright

In #294, we accidentally checked files with the default Go project
copyright header. Avoid this in the future with a copyright test.

copyright_test.go

@@ -0,0 +1,57 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package gosdk
+
+import (
+	"fmt"
+	"go/parser"
+	"go/token"
+	"io/fs"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+func TestCopyrightHeaders(t *testing.T) {
+	var re = regexp.MustCompile(`Copyright \d{4} The Go MCP SDK Authors. All rights reserved.
+Use of this source code is governed by an MIT-style
+license that can be found in the LICENSE file.`)
+
+	err := filepath.WalkDir(".", func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+
+		// Skip directories starting with "." or "_", and testdata directories.
+		if d.IsDir() && d.Name() != "." &&
+			(strings.HasPrefix(d.Name(), ".") ||
+				strings.HasPrefix(d.Name(), "_") ||
+				filepath.Base(d.Name()) == "testdata") {
+
+			return filepath.SkipDir
+		}
+
+		// Skip non-go files.
+		if !strings.HasSuffix(path, ".go") {
+			return nil
+		}
+
+		// Check the copyright header.
+		f, err := parser.ParseFile(token.NewFileSet(), path, nil, parser.ParseComments|parser.PackageClauseOnly)
+		if err != nil {
+			return fmt.Errorf("parsing %s: %v", path, err)
+		}
+		if len(f.Comments) == 0 {
+			t.Errorf("File %s must start with a copyright header matching %s", path, re)
+		} else if !re.MatchString(f.Comments[0].Text()) {
+			t.Errorf("Header comment for %s does not match expected copyright header.\ngot:\n%s\nwant matching:%s", path, f.Comments[0].Text(), re)
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+}