[MM-54730] Don't use old hardcoded rule for validating imported posts (#25823)

* don't use old hardcoded rule for validating imported posts

* fix http verb in doc

* Use client config

* Handle local mode

* E2E tests

* Enforce default if unable to use real limit

* Unit tests

* Fix tests

* Use model.PostMessageMaxRunesV2 as lower default

* Update direct post message length validation

---------

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: streamer45 <cstcld91@gmail.com>

server/cmd/mmctl/client/client.go

@@ -88,6 +88,7 @@ type Client interface {
 	MoveCommand(ctx context.Context, teamID string, commandID string) (*model.Response, error)
 	DeleteCommand(ctx context.Context, commandID string) (*model.Response, error)
 	GetConfig(ctx context.Context) (*model.Config, *model.Response, error)
+	GetOldClientConfig(ctx context.Context, etag string) (map[string]string, *model.Response, error)
 	UpdateConfig(context.Context, *model.Config) (*model.Config, *model.Response, error)
 	PatchConfig(context.Context, *model.Config) (*model.Config, *model.Response, error)
 	ReloadConfig(ctx context.Context) (*model.Response, error)