fix(AppCheck): Fixed getToken promise not being cleared

[ishowta]

Fix #6734

Immutable assignment by setState failed to clear the exchangeTokenPromise by assigning undefined to it.

firebase-js-sdk/packages/app-check/src/internal-api.ts

Line 126 in e2a90bf

state.exchangeTokenPromise = undefined;

There may be a better way, but I wrapped the exchangeTokenPromise in an object to prevent it from deep copy due to spread assignments.

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[hsubox76]

Just wanted to double check, were you able to put this code into the buggy app and observe that it fixed the bug you were seeing?

[ishowta]

I'm sorry, but we do not do that.

[hsubox76]

So thanks for pointing out that state setting is inconsistently done in this codebase, I've decided to do a more systematic fix of it with #6740 since I think it would be a bad idea to mix in more mutation. I added your tests into that PR and it passes, so if the test is a good replication of your situation it should fix the bug. Feel free to take a look at that PR and let me know what you think.