[BUG] Vertex AI Google Auth fails in certain circumstances (e.g. AWS multi cloud federation)

[eddietokens]

Basic checks

• I searched existing issues - this hasn't been reported
• I can reproduce this consistently
• This is a RubyLLM bug, not my application code

What's broken?

This could arguably be a Google Auth bug, but nevertheless there seems to be a slightly more robust way to apply the auth headers that works with both vertexi google SA access keys and more complex ADC setups that use IAM instance roles / multi cloud federation with AWS. Wonder if this might fix other things too.

How to reproduce

Not easy, can only repro in a AWS federated to Gcloud env .

Expected behavior

Works w/simple ADC credentials (e.g. service accounts w/access keys) and also w/federated auth (more complex usage of Google Auth)

What actually happened

@authorizer.fetch_access_token!['access_token'] does not populate the bearer token - empty.

Environment

AWS + GCP Iam instance role federated ADC.

[eddietokens]

Will create a PR soon but here's a gist monkey patch if anyone is stuck - "apply" seems to be more robust then the fetch access key call:

https://gist.github.com/eddietokens/96d3a26c91848267d50e9e8cd3f4fe30

[eddietokens]

We may also need to support providers as plugin gems in the future, as the implicit dependency on Google Auth and the inline "require" within the vertex provider is a bit brittle, but know why it was done.

[crmne]

If apply works, I'm happy to accept a simple PR with it.

No, we're not gonna make separate gems for each provider.