internal/lsp/source: add Vendor to list of supported commands

I wanted to add a regtest for this, but then immediately got stumped on
how to do it well. The best I've got is to add a helper that wraps
the client's call to executeCommand, but it's used in so many places that
an easier fix was to put it in executeCommand itself.

Fixes golang/go#40101

Change-Id: Iafb804fb77e57e6ac01e3de1d115058b21ac1954
Reviewed-on: https://go-review.googlesource.com/c/tools/+/241984
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

internal/lsp/command.go

@@ -6,6 +6,7 @@ package lsp
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"strings"
 
@@ -17,6 +18,16 @@ import (
 )
 
 func (s *Server) executeCommand(ctx context.Context, params *protocol.ExecuteCommandParams) (interface{}, error) {
+	var found bool
+	for _, command := range s.session.Options().SupportedCommands {
+		if command == params.Command {
+			found = true
+			break
+		}
+	}
+	if !found {
+		return nil, fmt.Errorf("unsupported command detected: %s", params.Command)
+	}
 	switch params.Command {
 	case source.CommandTest:
 		unsaved := false

internal/lsp/source/options.go

@@ -102,11 +102,12 @@ func DefaultOptions() Options {
 				Sum: {},
 			},
 			SupportedCommands: []string{
+				CommandGenerate,
+				CommandRegenerateCgo,
 				CommandTest,
 				CommandTidy,
 				CommandUpgradeDependency,
-				CommandGenerate,
-				CommandRegenerateCgo,
+				CommandVendor,
 			},
 		},
 		UserOptions: UserOptions{