feat(ai): add VertexAI provider for Claude models

Add support for Google Cloud Vertex AI as a provider to access Claude models
using Application Default Credentials (ADC). This allows users to route their
Fabric requests through Google Cloud Platform instead of directly to Anthropic,
enabling billing through GCP.

Features:
- Support for Claude models (Sonnet 4.5, Opus 4.5, Haiku 4.5, etc.) via Vertex AI
- Uses Google ADC for authentication (no API keys required)
- Configurable project ID and region (defaults to 'global' for cost optimization)
- Full support for streaming and non-streaming requests
- Implements complete ai.Vendor interface

Configuration:
- VERTEXAI_PROJECT_ID: GCP project ID (required)
- VERTEXAI_REGION: Vertex AI region (optional, defaults to 'global')

Closes #1570

go.sum

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