### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

When calling the `/v1/messages/count_tokens` endpoint with a system prompt against a Vertex AI Claude model (e.g. `vertex_ai/claude-sonnet-4-6`), the returned `input_tokens` value is identical to the value returned without a system prompt. The system prompt tokens are silently not counted.

The same request against `azure_ai/claude-sonnet-4-6` and `bedrock/global.anthropic.claude-sonnet-4-6` correctly accounts for the system prompt and returns a higher `input_tokens` value when a system prompt is included.

**Expected behaviour** `input_tokens` reflects tokens from both messages and system.

**Actual behaviour** `input_tokens` reflects only messages tokens; system is silently ignored.

**Root Cause (code-level)**

The system parameter is dropped at the Vertex AI partner-model layer. The call chain is:

1. `VertexAITokenCounter.count_tokens` (`litellm/llms/vertex_ai/common_utils.py:1087`) receives system but does not forward it to the next layer:

```python result = await partner_models_handler.count_tokens( model=model_to_use, messages=…