chore: Fix resource group header

[tomfrenken]

Context

AI/gen-ai-hub-sdk-js-backlog#146.

A user discovered a bug where we do not properly set the resource group after they have set it in the client's initialization.

With this fix, this should be fixed.

Definition of Done

• Code is tested (Unit, E2E)
• Error handling created / updated & covered by the tests above
• Documentation updated
• (Optional) Aligned changes with the Java SDK
• (Optional) Release notes updated

[tomfrenken]

@reviewers I intentionally didn't add tests for all the other clients yet, I would copy the test over to all affected clients, so please let me know if the current way I test it is sufficient before I copy it over.

[MatKuhr]

duplicating these tests a few times is of course a bit annoying. Is there any way to shorten them perhaps?

Pseudo code:

const expectedEndpoint = { url: 'localhost', resourceGroup: 'default' }
const mock = mock(httpClient)
client.run(prompt)
expect(mock.executeRequest).toBeCalled().with(expectedEndpoint)

Since we don't care about the full flow, we only care about the correct RG being passed on..

[ZhongpinWang]

LGTM. Only one question came to my mind while reviewing the PR.

Also, I suppose we need a changeset for the fix? I think we can add this to the PR template later because this can be missed so easily.

[ZhongpinWang]

Just in case, don't forget the changeset.