[Voltagent] Expose HTTP request headers in dynamic agent model/tool context

### Is your feature request related to a problem? Please describe. When using a VoltAgent instance, dynamic agents allow `model` and `tools` to be functions that receive `context`, but they do not have direct access to the underlying HTTP request headers. This forces us to create a manual proxy layer that reads headers (e.g., auth tokens, tenant IDs, custom headers) and copies them into `request.body.options.context` so that: - `model: ({ context }) => ... ` - `tools: ({ context }) => ... ` can see them. Ideally, there would be a way to access request‑level headers (or some `headers` object) at the same scope as `context` during model and tool construction, so we don’t need to rewire every request through an ad‑hoc proxy. Use case: - 3rd‑party services call our VoltAgent endpoints and pass auth / custom headers. - Those headers are required to configure internal model providers and tools (e.g., X‑Tenant‑ID,  Authorization,  X‑User‑ID). ### Describe alternatives you've considered