fix(ingest/vertexai): pass credentials to project discovery client

[eylulyr]

Summary

VertexAI ingestion using project_id_pattern or project_labels fails at startup with DefaultCredentialsError, even when a service account credential block is configured in the recipe. Project discovery builds its Cloud Resource Manager client without credentials, so it falls back to Application Default Credentials, which are not present on the executor.

This PR threads the source's configured credentials into the discovery client,
matching the Dataplex source.

Root cause

VertexAISource._resolve_target_projects() called resolve_gcp_projects(filter_cfg, self.report) without a projects_client, so the helper constructed a bare ProjectsClient() that uses ADC. The configured credentials (already used for aiplatform.init() and MetadataServiceClient) were never applied to project discovery. The failure happens in init, before any extraction.

Fix

Pass a credentialed ProjectsClient to resolve_gcp_projects, built only when discovery is actually needed (project_ids is empty).

resolved_projects = resolve_gcp_projects(
    filter_cfg,
    self.report,
    projects_client=(
        ProjectsClient(credentials=self._credentials)
        if not self.config.project_ids
        else None
    ),
)

This is exactly what the Dataplex source already does in its _project_ids
resolution, and consistent with BigQuery passing explicit credentials to its GCP
clients. Only the VertexAI caller was missing it.

Testing

Two unit tests in tests/unit/vertexai/test_vertexai_source.py:

• Discovery path: asserts ProjectsClient is built with the source credentials and passed to resolve_gcp_projects.
• Explicit project_ids: asserts no client is built and None is forwarded.

Full VertexAI source suite passes (20 tests).

Checklist

• The PR conforms to DataHub's Contributing Guideline (Commit Message Format)
• Tests for the changes have been added/updated
• Docs related to the changes have been added/updated (no doc change needed —
  required IAM permissions are unchanged)

[github-actions]

Linear: ING-2950

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