Validate duplicate IDs for JS client

[levand]

Description of changes

Validate unique IDs when adding embeddings.

Test plan

PR includes unit tests.

Documentation Changes

No documentation changes required.

[levand]

@jeffchuber how do you want me to merge this?

This test is targeting team/hypothesis-tests, which is where the corresponding Python code already is.

But you requested in this PR review that I split the JS tests out to different files. That's only present in main.

Would it be ok if we merged this as-is, then handled moving the tests over when we eventually merge to main? That will keep things in order.