Sandbox does not forward GOOGLE_GENAI_API_VERSION into container

### What happened?

When `GEMINI_SANDBOX=true` is set and `GOOGLE_GENAI_API_VERSION` is used alongside `GOOGLE_GEMINI_BASE_URL` to construct a Vertex-compatible API path, the sandbox fails with:

```
ModelNotFoundError: Not found (404)
```

`sandbox.ts` explicitly forwards a hardcoded list of environment variables into the Docker container (e.g., `GOOGLE_GEMINI_BASE_URL`, `GOOGLE_VERTEX_BASE_URL`, `GOOGLE_GENAI_USE_VERTEXAI`). When support for `GOOGLE_GENAI_API_VERSION` was added in `contentGenerator.ts`, the corresponding update to `sandbox.ts` was missed. The variable is available outside the sandbox but silently dropped when entering it.

Without `GOOGLE_GENAI_API_VERSION`, a `GOOGLE_GEMINI_BASE_URL` that depends on the API version segment to form a complete URL becomes invalid, and the API returns a 404.

#### Example

```sh
export GOOGLE_GEMINI_BASE_URL="https://my-gateway/vertex/"
export GOOGLE_GENAI_API_VERSION="v1beta1/projects/my-project/locations/global/publishers/google"
export GEMINI_SANDBOX=true
gemini -p "hello"
# => ModelNotFoundError: Not found (404)
```

Unsetting `GEMINI_SANDBOX` makes it work.

### What did you expect to happen?

`GOOGLE_GENAI_API_VERSION` should be forwarded into the sandbox container like the other `GOOGLE_*` environment variables, so that custom Vertex-compatible base URLs work correctly in sandbox mode.

### Client information

Client Information

Run `gemini` to enter the interactive CLI, then run the `/about` command.

```console
> /about

│ About Gemini CLI │
│ │
│ CLI Version 0.32.1 │
│ Git Commit 4d9d0b3 │
│ Model Auto (Gemini 3) │
│ Sandbox sandbox-0.32.1-0 │
│ OS linux │
│
```

### Login information

API key

### Anything else we need to know?

The environment variable was introduced in https://github.com/google-gemini/gemini-cli/pull/16177 to address [#16173](https://github.com/google-gemini/gemini-cli/issues/16173), however a small change in `sandbox.ts` is missing.

**Fix:**
Add `GOOGLE_GENAI_API_VERSION` to the forwarded env vars in `packages/cli/src/utils/sandbox.ts`, in both the Docker path ([~line 516](https://github.com/google-gemini/gemini-cli/blob/4c5e887732a24870b41d1028b1ed294493ad9033/packages/cli/src/utils/sandbox.ts#L516-L536)) and the LXC `envVarsToForward` record ([~line 957](https://github.com/google-gemini/gemini-cli/blob/4c5e887732a24870b41d1028b1ed294493ad9033/packages/cli/src/utils/sandbox.ts#L957)).

I have a fix in my fork but am unable to open a pull request due to the CLA requirement and some build issue. Anyone interested in submitting the PR is welcome to take the change from my commit:
https://github.com/google-gemini/gemini-cli/commit/d00e8b5417f998fc07a8abe5721807374186f876

---

### Workaround

Callers can use the `SANDBOX_ENV` escape hatch to forward the variable manually (e.g., `SANDBOX_ENV=GOOGLE_GENAI_API_VERSION=`). This is not ideal because it requires every caller to know about this sandbox implementation detail and shifts the burden of working around an upstream bug to downstream consumers.