perf(llc): skip replaying oversized /sync payloads

[xsahil03x]

Replaying a large /sync payload through handleEvent holds local persistence and state updates long enough to slow down the regular requests that need them. Payloads over 250 events are no longer replayed — the synced channels are re-queried in their place.

Implements the Sync events limit spec, matching stream-chat-swift (SyncRepository) and stream-chat-android (SyncManager), which use the same 250-event threshold.

Ticket: FLU-756

What changed

• StreamChatClient.sync skips event replay when a /sync response carries more than 250 events.
• On reconnect the synced channels are re-queried with queryChannels in their place, one 30-channel page at a time (the server returns at most 30 channels per request, so a single query would silently cover only the first page).
• lastSyncAt advances only once that refresh succeeded. Dropping the events is safe when their state has been re-fetched; advancing past a failed refresh would lose them.
• Mark-all-read events from a skipped payload are still applied, since a channel refresh does not carry read state.
• lastSyncAt fallbacks now use DateTime.timestamp() instead of local time.

Behavior change worth a second look

On reconnect, the re-query after a skipped replay runs even when recoverStateOnReconnect is false (which StreamChatCore sets for every app built on stream_chat_flutter). Skipping replay drops in-memory channel state, so without it there is nothing to repair the currently open channel. When the refresh does run, the flag's own re-query is skipped so the channels are not queried twice.

A direct client.sync() call issues no queries — only the reconnect path opts into the refresh.

Verification

dart analyze --fatal-infos, dart format, and the full stream_chat suite (1632 tests) pass. New tests cover the skip, the re-query in its place, page batching past 30 channels, lastSyncAt being kept when the refresh fails, and mark-all-read surviving the skip.

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