Emit a single disconnected event when connecting fails

[hiroshihorie]

Summary

Follow-up to #1065. Ensures the SDK emits exactly one RoomDisconnectedEvent per failed connection attempt.

Previously a failed initial connect produced two EngineDisconnectedEvents, and therefore two RoomDisconnectedEvents and two room cleanups:

1. The signal client emits SignalDisconnectedEvent(signalingConnectionFailure) while connect() is failing (the validate path has done this since Improve reconnect logic for websocket #406, and the certificate pinning path in Certificate pinning #1065 follows the same pattern), which the engine relayed as EngineDisconnectedEvent.
2. Engine.connect's catch then emits EngineDisconnectedEvent(joinFailure) for the same failure.

Changes

• Engine.connect's catch is now the single emitter for initial connect failures. It picks the reason by error type: signalingConnectionFailure for CertificatePinningException, joinFailure otherwise.
• The signalingConnectionFailure relay in the engine's signal listener is removed (with a comment explaining why). During reconnects the engine's reconnect handling already owns disconnect emission, so the relay only ever produced duplicates. This also removes the _isReconnecting/_attemptingReconnect guard that Certificate pinning #1065 added to suppress the relay during reconnects, since there is no longer anything to suppress.
• SignalDisconnectedEvent at the signal level is unchanged, only the engine-level relay is removed.

Behavior change

Apps listening for RoomDisconnectedEvent now receive one event per failed connect instead of two. The reasons are unchanged for the common case (joinFailure); certificate pinning failures surface as signalingConnectionFailure. Apps that specifically depended on receiving both events for a single failure would see one.

Testing

Added room-level tests through the mock e2e container asserting exactly one RoomDisconnectedEvent per failed initial connect, for both a certificate pinning failure (signalingConnectionFailure) and a generic websocket failure (joinFailure). Both tests fail against the previous code (two events observed) and pass with this change.

[devin-ai-integration]

Devin Review found 1 potential issue.

[devin-ai-integration]

🔍 Region fallback in Room.connect may emit a spurious RoomDisconnectedEvent before retrying

When Room.connect catches a WebSocketException or ConnectException from the first engine.connect call and falls back to a regional URL (room.dart:340-366), the engine's catch block at lib/src/core/engine.dart:271-275 has already emitted an EngineDisconnectedEvent. The room's listener at lib/src/core/room.dart:596-601 converts this into a RoomDisconnectedEvent. If the second engine.connect succeeds, the room ends up having emitted a disconnect event followed by a successful connection — which could confuse listeners. This is a pre-existing issue not introduced by this PR, but since the PR's goal is to ensure a single disconnected event per failure, it may be worth addressing in a follow-up.

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