fix(client): actually unregister the requested plugin channel

[GoatTech-42]

Fixes #1534.

unregisterChannel() used Array.find() and stored the matching channel name in a variable called index, then passed that string to splice():

const index = channels.find(function (name) {
  return channel === name
})
if (index) {
  proto.types[channel] = undefined
  channels.splice(index, 1)

JavaScript coerces a non-numeric channel string to 0, so unregistering any custom channel removed the first registered channel (usually the built-in minecraft:register) and left the requested channel active. The truthiness check would also have skipped a legitimate match at index 0 once findIndex was used.

The fix uses findIndex() and tests for -1:

const index = channels.findIndex(function (name) {
  return channel === name
})
if (index !== -1) {

Regression tests in test/pluginChannelsTest.js cover both failure modes: the requested channel no longer receives custom_payload events after being unregistered (previously the splice removed the wrong entry, so it stayed active), and a channel sitting at index 0 can be unregistered.