🐛 Mark initMessagingChannel timeout as expected error

[rcebulko]

This PR is a follow-up to #27247

That PR attempted to mark the timeout error as expected by checking that the error message raised by timeoutPromise was the opt_message provided. What I missed at the time was that getChannelError prepends text to the error message, so the equality check failed and didn't actually mark the error as expected.

This PR updates the check to instead verify that the error reason matches the opt_message provided to timeoutPromise