Make registerTool() return a promise

[beaufortfrancois]

This PR makes the registerTool() method return a Promise; fixes #175.

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Preview | Diff

[domfarolino]

I pushed some small fixes up:

• Preserving then, per https://github.com/whatwg/html?tab=contributing-ov-file#common-mistakes-around-prose-style which I try and follow in all my specs
• Fixed the parallelism issue. Before 4d6c78b, this PR broke the assert in https://webmachinelearning.github.io/webmcp/#notify-documents-of-a-tool-change, and that commit fixes it by adjusting the assert. This is right, since we need to queue a task to resolve the promise from an in parallel context, in order to make the Promise resolution well-ordered with the toolchange events.
• Runs the unregistration steps from in parallel too, which is now required since they assert/expect that.

Can you take a look @beaufortfrancois? If it looks good to you, I think we can merge.

[beaufortfrancois]

@domfarolino Thanks for the fixes!