feat: support token/project/location in provider options config

[pnancarrow]

Summary

• Allows setting token, project, and location via the JSON config's provider.options instead of requiring environment variables
• Config options take precedence over env vars, matching the pattern used by the google-vertex loader
• Changes loader signature from async () => to async (provider) => to access provider config

This enables fully self-contained managed deployments where a single config file configures everything:

"provider": {
  "google-vertex-anthropic": {
    "options": {
      "token": "my-bearer-token",
      "project": "my-gcp-project",
      "location": "us-east5",
      "baseURL": "https://my-gateway.example.com/..."
    }
  }
}

Context

We validated the original PR (anomalyco#14177) works for bypassing ADC auth via VERTEX_ANTHROPIC_TOKEN. However, for managed deployments (e.g. system-level config at /Library/Application Support/opencode/opencode.json), requiring users to also set an env var is friction. This change makes the config fully self-contained.

Test plan

• Added test: reads token from provider.options.token in config
• Added test: options.token takes precedence over VERTEX_ANTHROPIC_TOKEN env var
• Added test: reads project and location from provider.options

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[github-actions]

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