[MM-62924] WebSocketClient reconnection unit tests (#30135)

This commit adds some reconnection-specific unit tests to the WebSocketClient class, and fixes a few minor bugs:

- If `close()` is called during a reconnection delay, the socket won't close.
- If `send()` is called during a reconnection delay, we immediately try to reconnect (and don't respect any configured delays).
- If `initialize()` is called during a reconnection delay, we immediately try to reconnect (and don't respect any configured delays).
- If we receive two disconnection events, we'll attempt to reconnect with two new connections.

To allow for testing, I also needed to make the WebSocketClient more configurable. Specifically, the config allows for mocking the underlying websockets, and to control timeout delays.

webapp/channels/src/components/post_view/post_body_additional_content/__snapshots__/post_body_additional_content.test.tsx.snap

@@ -99,6 +99,13 @@ exports[`PostBodyAdditionalContent with a normal link Should render the plugin c
       WebSocketClient {
         "closeCallback": null,
         "closeListeners": Set {},
+        "config": Object {
+          "maxWebSocketFails": 7,
+          "maxWebSocketRetryTime": 300000,
+          "minWebSocketRetryTime": 3000,
+          "newWebSocketFn": [Function],
+          "reconnectJitterRange": 2000,
+        },
         "conn": null,
         "connectFailCount": 0,
         "connectionId": "",
@@ -114,6 +121,7 @@ exports[`PostBodyAdditionalContent with a normal link Should render the plugin c
         "postedAck": false,
         "reconnectCallback": null,
         "reconnectListeners": Set {},
+        "reconnectTimeout": null,
         "responseCallbacks": Object {},
         "responseSequence": 1,
         "serverHostname": "",
@@ -147,6 +155,13 @@ exports[`PostBodyAdditionalContent with a normal link Should render the plugin c
       WebSocketClient {
         "closeCallback": null,
         "closeListeners": Set {},
+        "config": Object {
+          "maxWebSocketFails": 7,
+          "maxWebSocketRetryTime": 300000,
+          "minWebSocketRetryTime": 3000,
+          "newWebSocketFn": [Function],
+          "reconnectJitterRange": 2000,
+        },
         "conn": null,
         "connectFailCount": 0,
         "connectionId": "",
@@ -162,6 +177,7 @@ exports[`PostBodyAdditionalContent with a normal link Should render the plugin c
         "postedAck": false,
         "reconnectCallback": null,
         "reconnectListeners": Set {},
+        "reconnectTimeout": null,
         "responseCallbacks": Object {},
         "responseSequence": 1,
         "serverHostname": "",