[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/mocks/client_mock.go

@@ -958,6 +958,22 @@ func (mr *MockClientMockRecorder) GetOAuthApps(arg0, arg1, arg2 interface{}) *go
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetOAuthApps", reflect.TypeOf((*MockClient)(nil).GetOAuthApps), arg0, arg1, arg2)
 }
 
+// GetOldClientConfig mocks base method.
+func (m *MockClient) GetOldClientConfig(arg0 context.Context, arg1 string) (map[string]string, *model.Response, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetOldClientConfig", arg0, arg1)
+	ret0, _ := ret[0].(map[string]string)
+	ret1, _ := ret[1].(*model.Response)
+	ret2, _ := ret[2].(error)
+	return ret0, ret1, ret2
+}
+
+// GetOldClientConfig indicates an expected call of GetOldClientConfig.
+func (mr *MockClientMockRecorder) GetOldClientConfig(arg0, arg1 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetOldClientConfig", reflect.TypeOf((*MockClient)(nil).GetOldClientConfig), arg0, arg1)
+}
+
 // GetOutgoingWebhook mocks base method.
 func (m *MockClient) GetOutgoingWebhook(arg0 context.Context, arg1 string) (*model.OutgoingWebhook, *model.Response, error) {
 	m.ctrl.T.Helper()