refactor(tests): annotate offline-support helpers

Type annotations for the two large integration-test helpers. No
behavior changes.

- offline-feature.tsx: narrow `_fiber` access via
  `as unknown as { _fiber: { pendingProps: { testID: string } } }` at
  13 call sites; rename `userId` → `user_id` in `generateMessage`
  options (snake_case per SDK); type `ChannelSort` literals properly
  (`last_updated: 1`); narrow `tablesInDb`, `JSON.parse` args, and
  `new Date(...)` args from `unknown`; import `ChannelMemberResponse`,
  `ChannelSort`, `Event`, `MessageResponse`, `ReactionResponse`,
  `ChannelAPIResponse` as `type` imports.
- optimistic-update.tsx: local wrapper cast
  `Channel as unknown as ComponentType<... & { initialValue?: string }>`
  to preserve the legacy `initialValue` prop the test passes; typed
  empty accumulators (`const newLatestReactions: ReactionResponse[]`);
  non-null assertion on `newReaction.user!.id`; cast
  `doUpdateMessageRequest` callbacks whose test-return shape doesn't
  include SDK-required `duration` etc.

Latent test bugs preserved (passing undeclared fields at runtime
that the SDK types don't include — `cid` on `ChannelMemberResponse`
and `GeneratedChannelResponseCustomValues`, `last_read` instead of
`last_read_at` on Event). Widened locally with
`as unknown as T & { cid: string }` etc. rather than changing test
data. These are follow-up items.

Takes test:typecheck 69 → 0. Full test suite: 752 passed, 14
skipped; only the pre-existing SQLite-isolation flake in
offline-support/index.test.ts fails. No regressions.

Author: oliverlaz