feat(google-vertex): update model YAMLs [bot]

[harshiv-26]

Auto-generated by poc-agent for provider google-vertex.

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Note

Low Risk
Low risk configuration-only change: updates model registry YAMLs with pricing, capabilities, and limits, which may affect model selection/feature gating but does not change runtime logic.

Overview
Updates Google Vertex AI Grok model definitions (grok-4.1-fast-* and grok-4.20-non-reasoning) from placeholder entries to full metadata.

Adds token pricing (including cache read costs), declared features (e.g., function_calling, structured_output, prompt_caching, system_messages), context window limits, supported text/image modalities, documentation sources, and marks models as preview; the reasoning variant is flagged with thinking: true.

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