feat(helm): add environment variables to support open source models for ab testing

[dembrane-sam-bot]

What this changes

• Support for passing GCP Service Account credentials and optional direct API keys for open-source model configurations (DeepSeek, Mistral Large, etc.) via Helm values and templates — _helpers.tpl, values-extended-env.yaml
• Added non-sensitive Helm environment configurations to allow controlling model routing, vertex location, vertex project, and api base defaults — values-extended-env.yaml

Cross-references:

• Companion dashboard A/B testing implementation on dembrane/echo: feat(dashboard): set up summary preference ab test under conversation overview (ECHO-820) echo#590

Confidence: High. All additions follow the exact pattern used for other LLM providers in the Helm charts.

[linear]

ECHO-820