Route Anthropic models on Vertex AI through the native endpoint

[dgageot]

Summary

Fixes #2469.

Claude models on Google Cloud's Vertex AI Model Garden do not support the OpenAI-compatible `/chat/completions` endpoint and fail with:

`FAILED_PRECONDITION: The deployed model does not support ChatCompletions.`

When `provider_opts.publisher == "anthropic"`, requests are now routed through the Anthropic-native `:rawPredict` / `:streamRawPredict` endpoints (with the `vertex-2023-10-16` body schema) using the `anthropic-sdk-go/vertex` subpackage, authenticated via Google Application Default Credentials. Other Model Garden publishers (`meta`, `mistral`, ...) continue to use the OpenAI-compatible path, and Gemini-on-Vertex is unchanged (it never enters this code path).

Example config

```yaml
models:
claude-on-vertex:
provider: google
model: claude-sonnet-4-20250514
provider_opts:
project: my-gcp-project
location: us-east5
publisher: anthropic
```

Routing

Config	Path
`provider: google`, publisher unset or `google`	`gemini.NewClient` (unchanged)
`provider: google`, `publisher: anthropic`	`anthropic.NewVertexClient` ( `:streamRawPredict` ) — new
`provider: google`, `publisher: meta` / `mistral` / ...	OpenAI-compatible endpoint (unchanged)

Changes

• `pkg/model/provider/anthropic/vertex.go` (new): `NewVertexClient` constructor that builds an Anthropic SDK client wired through `vertex.WithCredentials`. Credentials are resolved up front via `google.FindDefaultCredentials` so a missing ADC returns a friendly error. Explicitly clears `ANTHROPIC_API_KEY` (see in-code comment — this is required, not cosmetic).
• `pkg/model/provider/vertexai/modelgarden.go`: single `NewClient` entry point that dispatches on publisher and returns a small `Client` interface satisfied by both `anthropic.Client` and `openai.Client`. Shared `resolveProjectLocation` helper with URL-injection-safe validation (regex blocks `..`, `/`, uppercase, etc.).
• `pkg/model/provider/provider.go`: one-line dispatch into `vertexai.NewClient`.
• Tests: cover publisher extraction, project/location resolution (env-var fallback, `${VAR}` expansion, URL-injection attempts, uppercase rejection).
• Docs: `docs/providers/google/index.md` explains the two endpoint paths.
• `go.mod` / `go.sum`: transitive `google.golang.org/api` dependency picked up by `go mod tidy` from the `anthropic-sdk-go/vertex` import.

Validation

• `go build ./...` — clean.
• `mise lint` — 0 issues (`golangci-lint` + custom cop over 755 files + `go mod tidy --diff` all clean).
• `mise test` — full suite passes, including 11 new test cases covering URL-injection attempts.

Assisted-By: docker-agent