@spec get_gcp_token(map()) :: {:ok, String.t()} | {:error, any()}

Gets an access token for Google Cloud Platform authentication.

Supports multiple authentication methods:

• Direct access token (explicit token provided)
• Service account key data (JSON map)
• Service account key file (path to JSON file)
• Application Default Credentials (ADC):
  • Cloud Run/GCE metadata service (production)
  • gcloud CLI (local development)