fix(core): Fix backwards pagination not working if channel was never opened

[VelikovPetar]

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Linear: FLU-547

Github Issue: #

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Description of the pull request

When a user enters a channel with unread_count > 0 but the server-side read state has last_read_message_id = null and last_read = 0001-01-01T00:00:00Z (Go's time.Time{} zero value — the "never explicitly read" sentinel), StreamChannel._maybeInitChannel was unconditionally calling loadChannelAtTimestamp(currentUserRead.lastRead).

The backend silently ignores zero-time created_at_around (via IsZero()) and returns the channel tail. The client then mis-infers boundaries via _inferBoundariesFromAnchorTimestamp: because year-1 is before every loaded message, it concludes endOfPrependReached: true, sets _topPaginationEnded = true, and backwards pagination is permanently dead on the channel.

The fix guards the timestamp branch on lastRead.toUtc().isAfter(DateTime.utc(1970, 1, 1)). When it's the zero sentinel, the code falls through to the existing catch-all if (channel.state?.isUpToDate == false) loadChannelAtMessage(null) — i.e. stay on cached latest if up-to-date, otherwise reload latest.

Adds a _maybeInitChannel test group with three regression guards: idAround for lastReadMessageId, createdAtAround for real timestamps, and no query for the Go zero-time case.

Screenshots / Videos

Before	After
pagin-before.mp4	pagin-after.mp4

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