feat(Google Vertex Node): Add Application Default Credentials (ADC) authentication support

[MojRoid]

Summary

This PR adds Application Default Credentials (ADC) authentication support for Google Vertex AI nodes, enabling seamless authentication when running n8n in Google Cloud environments (GKE, Cloud Run, Compute Engine) or when using the gcloud CLI locally.

Changes:

• Add new googleApiAdcApi credential type for ADC authentication
• Add authentication selector to both Google Vertex Chat Model and Embeddings Google Vertex nodes
• Support project ID configuration via:
  • Node parameter override
  • Credential configuration
  • Auto-detection from ADC environment
• Extract shared Google Cloud regions to a reusable module (packages/nodes-base/credentials/common/google/regions.ts)
• Add comprehensive unit tests for both authentication methods
• Add documentation for ADC setup and usage

How to Test:

1. Create a new Google Vertex Chat Model or Embeddings Google Vertex node
2. Select "Application Default Credentials (ADC)" as the authentication method
3. Configure the ADC credential (optionally specify region and project ID)
4. If running locally, ensure you've run gcloud auth application-default login
5. If running in GCP, the node should automatically use the attached service account

Before:
Only Service Account JSON key authentication was supported

After:
Users can choose between Service Account or ADC authentication

Related Linear tickets, Github issues, and Community forum posts

Review / Merge checklist

• PR title and summary are descriptive. (conventions)
• Docs updated or follow-up ticket created.
• Tests included.
• PR Labeled with release/backport (if the PR is an urgent fix that needs to be backported)

[CLAassistant]

All committers have signed the CLA.

[cubic-dev-ai]

No issues found across 9 files

[n8n-assistant]

Hey @MojRoid,

Thank you for your contribution. We appreciate the time and effort you’ve taken to submit this pull request.

Before we can proceed, please ensure the following:
• Tests are included for any new functionality, logic changes or bug fixes.
• The PR aligns with our contribution guidelines.

Regarding new nodes:
We no longer accept new nodes directly into the core codebase. Instead, we encourage contributors to follow our Community Node Submission Guide to publish nodes independently.

If your node integrates with an AI service that you own or represent, please email [email protected] and we will be happy to discuss the best approach.

About review timelines:
This PR has been added to our internal tracker as "GHC-5823". While we plan to review it, we are currently unable to provide an exact timeframe. Our goal is to begin reviews within a month, but this may change depending on team priorities. We will reach out when the review begins.

Thank you again for contributing to n8n.