fix(network): RawWebSocketClientImpl.connect never awaits channel.ready - isConnected is true for a dead socket and the failure escapes unhandled

[koniz-dev]

Context

RawWebSocketClientImpl (lib/core/network/realtime/raw_websocket_client.dart,
untouched since its introducing commit 302490b) is the implementation the
provider doc tells adopters to inject for production
(lib/core/network/realtime/realtime_providers.dart:6-7: "Override this
provider to inject RawWebSocketClientImpl for production").

connect() (raw_websocket_client.dart:21-45) assigns
WebSocketChannel.connect(...) and returns. WebSocketChannel.connect
never throws for a network failure - the failure is reported through the
channel's ready future, which nothing awaits or handles. Two consequences,
both verified by running the class against a port that refuses connections:

$ dart --packages=.dart_tool/package_config.json scratchpad/ws_probe.dart
connect() returned normally: true
isConnected right after connect: true
Unhandled exception:
WebSocketChannelException: SocketException: Connection refused (OS Error: Connection refused, errno = 61), address = 127.0.0.1, port = 52118

(probe: await client.connect('ws://127.0.0.1:1'); print(client.isConnected);)

1. connect() reports success and isConnected reports true for a
   connection that never opened (isConnected is just _channel != null,
   :18). Calls to send() are then silently buffered into a dead sink.
2. The connection failure escapes as an unhandled async exception from
   the unawaited ready future. The try/catch around
   WebSocketChannel.connect at :28-44 can never catch it (the
   constructor does not throw for network errors), and the stream onError
   handler at :31-35 does not cover the ready future.

The existing test file
(test/core/network/realtime/raw_websocket_client_test.dart) never
exercises a connection attempt, so nothing guards this. An adopter who
follows the provider doc inherits a realtime client whose failure mode on an
unreachable server is a lying isConnected plus an unhandled exception.

No prior issue covers this surface (gh issue list --state all --search "websocket" and --search "realtime" return nothing about it).

Acceptance criteria

All driveable with flutter test on the host VM using dart:io sockets -
no device, no backend.

1. A test that calls connect() against a port that refuses connections
   (bind a ServerSocket on port 0, note the port, close it, connect)
   observes a defined, documented failure: either connect() completes
   with an error, or the error is delivered on stream - and in both cases
   no unhandled async error escapes (assert via runZonedGuarded or by the
   test simply not failing on an uncaught exception).
2. After that failed connect, isConnected is false.
3. A companion happy-path test using an in-process server
   (HttpServer + WebSocketTransformer) shows connect() completing,
   isConnected true, a send() arriving at the server, and messages
   from the server arriving on stream - proving the fix did not break the
   working path.
4. IRealtimeClient.connect's doc comment
   (lib/core/network/realtime/i_realtime_client.dart) states the error
   contract implementations must meet.