feat(providers): add Google Vertex AI provider for Claude

[eilonc-pillar]

Summary

Adds vertex-ai as a first-class provider, enabling Claude models via Google Cloud Vertex AI with Application Default Credentials (ADC).

• Uses AnthropicVertex from anthropic[vertex] SDK
• Reuses 100% of the existing Anthropic adapter (message conversion, tool handling, streaming)
• Only client instantiation differs — zero new adapters needed

Configuration

provider: vertex-ai
model: claude-opus-4-6

# Environment
export VERTEX_PROJECT=my-gcp-project   # required
export VERTEX_REGION=us-east5           # optional, default: us-east5

# Auth (any of these):
# 1. Attached service account on GCP VMs (ADC) — zero config
# 2. GOOGLE_APPLICATION_CREDENTIALS=/path/to/key.json
# 3. gcloud auth application-default login (local dev)

Changes

File	Lines	What
agent/anthropic_adapter.py	+58	build_vertex_client() + resolve_vertex_credentials()
agent/auxiliary_client.py	+28	_try_vertex() + auto-chain guard + aux model entry
hermes_cli/auth.py	+7	Provider registry entry
run_agent.py	+17	Provider detection + client init
pyproject.toml	+2	vertex optional extra

Tested

$ python3 -c "..."
Credentials: project=test-project, region=us-east5
Client: AnthropicVertex
Response: Hello to you!
Model: claude-opus-4-6
SUCCESS

Design Decisions

• No new adapter: AnthropicVertex speaks the same Messages API as Anthropic. Setting api_mode = "anthropic_messages" reuses all existing message conversion, tool handling, and streaming code.
• No beta headers: Beta headers are not forwarded to Vertex — feature availability may differ from the direct API. The SDK handles its own negotiation.
• Auto-chain guard: _resolve_api_key_provider() has an explicit vertex-ai guard to prevent the generic OpenAI client path from mishandling GCP project IDs.
• Credential fallback: Checks VERTEX_PROJECT, GOOGLE_CLOUD_PROJECT, and GCP_PROJECT_ID in order.

Install

pip install 'hermes-agent[vertex]'

[eilonc-pillar]

Hi all - @leon @intertwine @timknip @rewbs

Very good job on the Hermes agent! It delivers a lot of value and innovation.
I've opened this PR to enable powering hermes agent with Google's vertex AI juice (both ACP and SA json). Would appreciate if you consider to add this.

Best,
Eilon, Pillar Security