ADC Issue: Allow GCP ADC and cloud IAM auth without requiring static VERTEX_AI_API_KEY

[datasherlock]

Description

When configuring OpenKB to use Google Cloud Vertex AI (vertex_ai/gemini-3.8-flash) using standard GCP Application Default Credentials (ADC) rather than a static API key, I get the below error:

1. PageIndexError: API key not configured for provider 'vertex_ai'

• What happens: Running openkb add or initializing PageIndexClient triggers _validate_llm_provider in pageindex, which enforces a static VERTEX_AI_API_KEY env var.
• Expected behavior: Cloud providers that authenticate via ADC / IAM (vertex_ai) should be recognized as credentialed without demanding a static API key.
• Additionally, OpenKB's CLI prints a false Warning: No LLM API key found for these providers.

We have resolved this locally by:
Hooking PageIndexClient._validate_llm_provider to allow ADC/IAM cloud providers.
Updating _KEYLESS_OR_CLOUD_PROVIDERS in cli.py to prevent false missing-key warnings.