Update entrypoint and service to aimode in FxA auth flow

[punamdahiya]

Test step to use MLPA proxy server without API key

1. Set FxA server as stage by setting pref identity.fxaccounts.autoconfig.uri as https://accounts.stage.mozaws.net
2. Set pref browser.smartwindow.requireSignIn as true .
3. Set Smart Window endpoint as https://mlpa-dev.llm-proxy.nonprod.dataservices.mozgcp.net/v1/ and model as mistral-small-2503 and leave key pref empty
4. Click Switch to Smart Window and complete authentication flow. Please note your production FxA account will not work (though you can use same email address) and need to create an account on stage.
5. If prompted for API key , you can enter any text and continue

Noticed below edge case sometime
6. After successful authentication on staging server user is not redirected to Smart window and lands on account settings web page. If that happens restart build and try again to authenticate by clicking on 'Switch to Smart Window'

[mhammond]

this looks pretty-much exactly like I'd expect for a signed in user 👍

[mhammond]

I think you want a custom scope here, eg, "aimode" (probably as a const but I don't hate literals :) I think profile is likely to work, depending on how it is validated, but a specific scope is more correct. Even though the fxa api doesn't reflect this and should, this is a standard oauth "access token" and not really fxa specific (thinking towards enterprise etc :)

[punamdahiya]

Testing with scope as aimode not yet supported and fails to give a valid token

[LZoog]

Probably makes sense to update this to aiwindow in this PR, the entrypoint I believe is still TBD but the service has been updated on the FxA side to look for the service=aiwindow query param.