fix: subscriptions not closing after done

[nedsalk]

This fix was part of #1597 which didn't get merged. In essence, the piping done in

const subscriptionStreamReader = response
    .body!.pipeThrough(new FuelSubscriptionStream())
    .getReader();

was ignoring the value {value: undefined, done: true} which indicates a stream is done. This resulted in subscriptions stalling when they aren't cancelled externally with a break in a for-await-of loop even though their underlying connection got closed properly. I opted for not using piping for transforming streams but rather inlining the byte parsing in the next method of the iterator.

You can see more around the investigation of this problem in #1615 (comment).

Besides this, the PR also had a test that verified our code correctly handles keep-alive messages for long-running subscriptions which I added here as well.

[arboleya]

I like the new direction. 🫡

[github-actions]

Coverage Report:

Lines	Branches	Functions	Statements
79.43%(+0.01%)	70.17%(+0%)	77.91%(+0.01%)	79.51%(+0.01%)

Changed Files:

Ok	File (✨=New File)	Lines	Branches	Functions	Statements
🔴	packages/account/src/providers/fuel-graphql-subscriber.ts	91.3% (+1.3%)	62.5% (+0%)	100% (+0%)	91.3% (+1.3%)