MM-57018: support reattaching plugins (#26421)

* ProfileImageBytes for EnsureBotOptions

* leverage plugintest.NewAPI

* fix linting

* add UpdateUserRoles to plugin api

* MM-57018: support reattaching plugins

Expose a local-only API for reattaching plugins: instead of the server starting and managing the process itself, allow the plugin to be launched externally (eg within a unit test) and reattach to an existing server instance to provide the unit test with a fully functional RPC API, sidestepping the need for mocking the plugin API in most cases.

In the future, this may become the basis for running plugins in a sidecar container.

Fixes: https://mattermost.atlassian.net/browse/MM-57018

* drop unused supervisor.pid

* factor out checkMinServerVersion

* factor out startPluginServer

* restore missing setPluginState on successful reattach

* avoid passing around a stale registeredPlugin

* inline initializePluginImplementation

* have IsValid return an error

* explicitly close rpcClient

In the case of reattached plugins, the Unix socket won't necessarily disappear leaving the muxBrokers blocked indefinitely. And `Kill()` doesn't do anything if there's no process being managed.

* explicitly detachPlugin

* emphasize gRPC not being supported

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/plugin/health_check_test.go

@@ -52,7 +52,7 @@ func testPluginHealthCheckSuccess(t *testing.T) {
 
 	bundle := model.BundleInfoForPath(dir)
 	logger := mlog.CreateConsoleTestLogger(t)
-	supervisor, err := newSupervisor(bundle, nil, nil, logger, nil)
+	supervisor, err := newSupervisor(bundle, nil, nil, logger, nil, WithExecutableFromManifest(bundle))
 	require.NoError(t, err)
 	require.NotNil(t, supervisor)
 	defer supervisor.Shutdown()
@@ -93,7 +93,7 @@ func testPluginHealthCheckPanic(t *testing.T) {
 
 	bundle := model.BundleInfoForPath(dir)
 	logger := mlog.CreateConsoleTestLogger(t)
-	supervisor, err := newSupervisor(bundle, nil, nil, logger, nil)
+	supervisor, err := newSupervisor(bundle, nil, nil, logger, nil, WithExecutableFromManifest(bundle))
 	require.NoError(t, err)
 	require.NotNil(t, supervisor)
 	defer supervisor.Shutdown()