MM-57873 Check user visibility when accepting channel invites for DMs (#27331)

* check user visibility when accepting channel invites for DMs

* stronger visibility checking for DM users

* check for correct remoteid for remote user in DM invite

* fix unit test

---------

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

server/platform/services/sharedchannel/mock_AppIface_test.go

@@ -558,6 +558,36 @@ func (_m *MockAppIface) UpdatePost(c request.CTX, post *model.Post, safeUpdate b
 	return r0, r1
 }
 
+// UserCanSeeOtherUser provides a mock function with given fields: c, userID, otherUserId
+func (_m *MockAppIface) UserCanSeeOtherUser(c request.CTX, userID string, otherUserId string) (bool, *model.AppError) {
+	ret := _m.Called(c, userID, otherUserId)
+
+	if len(ret) == 0 {
+		panic("no return value specified for UserCanSeeOtherUser")
+	}
+
+	var r0 bool
+	var r1 *model.AppError
+	if rf, ok := ret.Get(0).(func(request.CTX, string, string) (bool, *model.AppError)); ok {
+		return rf(c, userID, otherUserId)
+	}
+	if rf, ok := ret.Get(0).(func(request.CTX, string, string) bool); ok {
+		r0 = rf(c, userID, otherUserId)
+	} else {
+		r0 = ret.Get(0).(bool)
+	}
+
+	if rf, ok := ret.Get(1).(func(request.CTX, string, string) *model.AppError); ok {
+		r1 = rf(c, userID, otherUserId)
+	} else {
+		if ret.Get(1) != nil {
+			r1 = ret.Get(1).(*model.AppError)
+		}
+	}
+
+	return r0, r1
+}
+
 // NewMockAppIface creates a new instance of MockAppIface. It also registers a testing interface on the mock and a cleanup function to assert the mocks expectations.
 // The first argument is typically a *testing.T value.
 func NewMockAppIface(t interface {