test: isolates temp test fixtures (#1569)

## Motivation and Context

Replace shared temp paths with `t.TempDir()`` so each test gets its own
cleanup-scoped directory.
- This avoids leftover files and cross-test interference. No production
code changed.

## How Has This Been Tested?

GitHub CI passed.
- https://github.com/yowainwright/registry/pull/1

## Breaking Changes

No breaking changes; just tests.

## Types of changes

- [x] chore

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

## Additional context

Small test-only cleanup. No production code paths are changed.

Co-authored-by: Radoslav Dimitrov <radoslav@stacklok.com>

cmd/publisher/commands/testutil_test.go

@@ -95,9 +95,7 @@ func SetupTestToken(t *testing.T, registryURL, token string) string {
 func CreateTestServerJSON(t *testing.T, serverJSON apiv0.ServerJSON) (string, string) {
 	t.Helper()
 
-	tempDir, err := os.MkdirTemp("", "mcp-publisher-test")
-	require.NoError(t, err)
-	t.Cleanup(func() { os.RemoveAll(tempDir) })
+	tempDir := t.TempDir()
 
 	jsonData, err := json.MarshalIndent(serverJSON, "", "  ")
 	require.NoError(t, err)

cmd/publisher/commands/validate_test.go

@@ -140,16 +140,13 @@ func TestValidateCommand_NoServerFile(t *testing.T) {
 	server := SetupMockRegistryServer(t, nil, nil)
 	SetupTestToken(t, server.URL, "test-token")
 
-	// Don't create server.json
-	tempDir, err := os.MkdirTemp("", "mcp-publisher-test")
-	require.NoError(t, err)
-	defer os.RemoveAll(tempDir)
+	tempDir := t.TempDir()
 
 	originalDir, err := os.Getwd()
 	require.NoError(t, err)
 	defer func() { _ = os.Chdir(originalDir) }()
 
-	_ = os.Chdir(tempDir)
+	require.NoError(t, os.Chdir(tempDir))
 
 	err = commands.ValidateCommand([]string{})
 
@@ -161,15 +158,13 @@ func TestValidateCommand_InvalidJSON(t *testing.T) {
 	server := SetupMockRegistryServer(t, nil, nil)
 	SetupTestToken(t, server.URL, "test-token")
 
-	tempDir, err := os.MkdirTemp("", "mcp-publisher-test")
-	require.NoError(t, err)
-	defer os.RemoveAll(tempDir)
+	tempDir := t.TempDir()
 
 	originalDir, err := os.Getwd()
 	require.NoError(t, err)
 	defer func() { _ = os.Chdir(originalDir) }()
 
-	_ = os.Chdir(tempDir)
+	require.NoError(t, os.Chdir(tempDir))
 
 	// Create invalid JSON file
 	err = os.WriteFile("server.json", []byte("{ invalid json }"), 0600)