Statically ensure that UserTopicVisibilityPolicy.unknown is not possible in our data models

[PIG208]

This was brought up here: #1041 (comment).

UserTopicVisibilityPolicy.unknown allows us to deserialize unknown enum values gracefully at the edge.

zulip-flutter/lib/model/channel.dart

Lines 216 to 222 in 9e42f26

	static bool _warnInvalidVisibilityPolicy(UserTopicVisibilityPolicy visibilityPolicy) {
	if (visibilityPolicy == UserTopicVisibilityPolicy.unknown) {
	// Not a value we expect. Keep it out of our data structures. // TODO(log)
	return true;
	}
	return false;
	}

zulip-flutter/lib/model/channel.dart

Lines 348 to 350 in 9e42f26

	if (_warnInvalidVisibilityPolicy(visibilityPolicy)) {
	visibilityPolicy = UserTopicVisibilityPolicy.none;
	}

However, while we manually declare that it wouldn't be possible that we encounter UserTopicVisibilityPolicy.unknown when accessing our internal data store (i.e.: ChannelStore), our type doesn't agree with that, and require the UserTopicVisibilityPolicy.unknown case to be handled for an exhaustive switch. We use assert in such cases, but it is not supposed to be reachable (unless we have a bug).

We should make sure that such an assertion is not needed. This might require removing unknown from UserTopicVisibilityPolicy.

We specifically exclude UserTopicVisibilityPolicy.unknown at the edge. This may or may not apply to other unknown enum values. It would be good to do a sweep of those and decide if we need to follow-up on them as well.

[RITWIZSINGH]

Hey there,
actually I am new to open source contribution
I want to ask how can i help towards solving this issue ?
Can you assign this issue to me?