功能请求: 为 Claude 模型添加原生 Google Vertex AI 提供程序

1. nanobot/providers/vertex_ai_provider.py (new, ~120 lines)

• VertexAIProvider(AnthropicProvider) that instantiates AsyncAnthropicVertex in __init__.
• Strips provider prefixes like google_vertex_ai/, vertex/, Anthropic/ from the model string.
• Helpers: get_adc_status(), login_vertex_ai() (runs gcloud auth application-default login), logout_vertex_ai() (runs gcloud auth application-default revoke).

1. nanobot/providers/factory.py — new branch:

elif backend == "google_vertex_ai":
    from nanobot.providers.vertex_ai_provider import VertexAIProvider
    provider = VertexAIProvider(
        default_model=model,
        project=getattr(p, "project", None) if p else None,
        region=getattr(p, "region", None) if p else None,
    )

1. nanobot/providers/registry.py — new ProviderSpec:

ProviderSpec(
    name="google_vertex_ai",
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