### Description

Following the implementation of drop_params stripping in the Anthropic pass-through endpoint (#31070), other experimental pass-through endpoints (such as Gemini, Cohere, and Vertex AI) still ignore the global \drop_params=True\ and \dditional_drop_params\ configurations.

### Proposed Solution

Implement similar parameter-stripping logic inside the respective pass-through routing/handlers for Cohere, Gemini, and Vertex AI. When \drop_params=True\ or \dditional_drop_params\ is configured on the router/proxy, the proxy should dynamically strip unsupported or explicitly-listed parameters before forwarding the request payload to the downstream provider endpoints. This keeps behavior consistent across all pass-through endpoints and avoids HTTP 400 errors from strict backend schema validations.