test(llc): migrate the stream_chat test suite to the stream_chat_test algebra

[VelikovPetar]

Important

Depends on #2950 (test(repo): add \stream_chat_test` package). This branch is built on feat/stream-chat-test-package, so the diff below carries that package (4 files, ~1.9k lines) in addition to the migration. Merge #2950 first — once it lands, this diff reduces to the migration alone. Both branches carry the latest master`.

Rewrites the stream_chat test suite on top of the chatClientTest / channelTest
harness from #2950, and deletes the three monolithic test files it replaces.

What changes

• channel_client_state_test.dart (164 tests) → 21 files under test/src/client/channel/state/
• channel_test.dart (186 tests) → 28 files under test/src/client/channel/
• client_test.dart (~200 tests) → 16 files as test/src/client/client_*_test.dart
• Old shared helpers retired: test/src/fakes.dart and test/src/matchers.dart deleted,
  test/src/mocks.dart shrunk to the seven doubles that live below the harness seam
  (HTTP client, Dio, logger, token/connection-id managers, WS channel/sink, general API).
• STYLE_GUIDE.md / TESTING.md document the harness and the three conventions it
  depends on (exact-argument stubbing, deterministic fixtures, UTC in emitted events).

The tests now run a real StreamChatClient with three seams replaced — REST API,
WebSocket transport, persistence — so event decoding, channel state, the retry queue,
the delivery reporter and reconnection all execute production code. Several tests
consequently reach a layer deeper than before: they stub api.message.sendMessage
rather than client.sendMessage, putting the client→API translation under test too.

Evidence

	master	this branch
tests passing	1,693	1,699
lib/ lines covered	6,753	6,817
lines covered on master only	—	3
verify(...).called(n)	280	299
verifyNoMoreInteractions	124	124 (identical per sub-API target)
any(...) wildcard matchers	753	590
unawaited expectLater	53	3
real network calls escaping the suite	2	0
wall time	1:17	0:42

• Test-name parity is exact: all 550 test names from the deleted files have a
  counterpart here; verified by runner-level enumeration, not by grep.
• The 3 remaining uncovered lines are ?? DateTime.now() / ?? Uuid().v4()
  default initializers in poll.dart and message_reminder.dart, no longer reached
  because fixtures pass ids and timestamps explicitly. That is the trade that makes
  equality-based mock matching deterministic.
• The 2 leaked network calls on master came from a test that built a client without a
  chatApi, sending a real request to chat.stream-io-api.com on every run. The default
  REST wiring those calls incidentally covered is now pinned by
  client_default_wiring_test.dart with a stubbed HttpClientAdapter.

Intentional deviations

• +6 tests: one pins the channel-with-persistence lifecycle (the old group had
  setUp/tearDown and zero tests); the rest cover channel.config, the auth interceptor's
  non-token-expiry passthrough, the default REST wiring, and the retry queue giving up
  when the policy declines.
• Four tests titled "non-retriable" actually used retriable errors; they now use
  non-retriable ones, matching their titles.
• channel.on asserted event identity, which held only because the event never left
  the process. Events now round-trip through real JSON decoding, so it asserts every
  field the event carries instead.
• client_reconnection_test.dart is a deliberate carve-out on a hand-rolled fake socket:
  the anonymous-token user id cannot pass the fake server's connect-URI validation, and
  the reconnect-recovery tests need to drop an established connection. Both are documented
  in the file, and a self-test in the harness fails the moment the gap closes.

Follow-ups (not in this PR)

• The harness's guarded zone drops async errors that arrive after the test body completes.
• ~12s of the suite is wall-clock sleeps straddling production debounce constants; a
  deterministic fix needs a seam to advance them.
• mockFailedAuth's token-expiry refresh branch is documented but not yet exercised.

Bugs this surfaced (pre-existing on master, filed separately)

• RetryQueue can re-send the same message an arbitrary number of times: Message.createdAt
  falls back to a fresh DateTime.now() on every read when neither timestamp is set, which
  makes the queue's heap comparator non-reflexive, so remove() silently fails.
• Expired live locations are never pruned from ChannelClientState.activeLiveLocations for
  messages outside the loaded window — locationExpiredResolver reroutes the event and the
  channel handler no-ops on a _findLocationMessage miss.

Both were invisible on master because its tests mocked StreamChatClient, so no event
resolvers or heap invariants ran.

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