feat: adding support digest pinning for flux plugin install

Signed-off-by: iam-karan-suresh <karansuresh.info@gmail.com>

cmd/flux/plugin_test.go

@@ -211,6 +211,7 @@ func TestParseNameVersion(t *testing.T) {
 		{"operator@0.45.0", "operator", "0.45.0"},
 		{"my-tool@1.0.0", "my-tool", "1.0.0"},
 		{"plugin@", "plugin", ""},
+		{"operator@sha256:abc123", "operator", "sha256:abc123"},
 	}
 
 	for _, tt := range tests {
@@ -226,6 +227,27 @@ func TestParseNameVersion(t *testing.T) {
 	}
 }
 
+func TestIsDigestRef(t *testing.T) {
+	tests := []struct {
+		input string
+		want  bool
+	}{
+		{"sha256:06e0a38db4fa6bc9f705a577c7e58dc020bfe2618e45488599e5ef7bb62e3a8a", true},
+		{"0.45.0", false},
+		{"", false},
+		{"sha256", false},
+		{"SHA256:abc", false}, // case-sensitive
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.input, func(t *testing.T) {
+			if got := isDigestRef(tt.input); got != tt.want {
+				t.Errorf("isDigestRef(%q) = %v, want %v", tt.input, got, tt.want)
+			}
+		})
+	}
+}
+
 func TestPluginDiscoverSkipsBuiltins(t *testing.T) {
 	origHandler := pluginHandler
 	defer func() { pluginHandler = origHandler }()