fix(plugins): clear plugin errors on startup to allow retrying

Plugins that entered an error state (e.g., incompatible with the
Navidrome version) would remain in that state across restarts, blocking
the user from retrying. This adds a ClearErrors method to
PluginRepository that resets the last_error field on all plugins, and
calls it during plugin manager startup before syncing and loading.

Signed-off-by: Deluan <deluan@navidrome.org>

tests/mock_plugin_repo.go

@@ -29,6 +29,20 @@ func (m *MockPluginRepo) SetError(err bool) {
 	m.Err = err
 }
 
+func (m *MockPluginRepo) ClearErrors() error {
+	if m.Err {
+		return errors.New("unexpected error")
+	}
+	for i := range m.All {
+		m.All[i].LastError = ""
+	}
+	for k, p := range m.Data {
+		p.LastError = ""
+		m.Data[k] = p
+	}
+	return nil
+}
+
 func (m *MockPluginRepo) SetData(plugins model.Plugins) {
 	m.Data = make(map[string]*model.Plugin, len(plugins))
 	m.All = plugins