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/model/client4.go

@@ -7352,6 +7352,36 @@ func (c *Client4) InstallMarketplacePlugin(ctx context.Context, request *Install
 	return &m, BuildResponse(r), nil
 }
 
+// ReattachPlugin asks the server to reattach to a plugin launched by another process.
+//
+// Only available in local mode, and currently only used for testing.
+func (c *Client4) ReattachPlugin(ctx context.Context, request *PluginReattachRequest) (*Response, error) {
+	buf, err := json.Marshal(request)
+	if err != nil {
+		return nil, NewAppError("ReattachPlugin", "api.marshal_error", nil, "", http.StatusInternalServerError).Wrap(err)
+	}
+	r, err := c.DoAPIPost(ctx, c.pluginsRoute()+"/reattach", string(buf))
+	if err != nil {
+		return BuildResponse(r), err
+	}
+	defer closeBody(r)
+
+	return BuildResponse(r), nil
+}
+
+// DetachPlugin detaches a previously reattached plugin.
+//
+// Only available in local mode, and currently only used for testing.
+func (c *Client4) DetachPlugin(ctx context.Context, pluginID string) (*Response, error) {
+	r, err := c.DoAPIPost(ctx, c.pluginRoute(pluginID)+"/detach", "")
+	if err != nil {
+		return BuildResponse(r), err
+	}
+	defer closeBody(r)
+
+	return BuildResponse(r), nil
+}
+
 // GetPlugins will return a list of plugin manifests for currently active plugins.
 func (c *Client4) GetPlugins(ctx context.Context) (*PluginsResponse, *Response, error) {
 	r, err := c.DoAPIGet(ctx, c.pluginsRoute(), "")