### Check for existing issues

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

### The Feature

Recently, with [#21560](https://github.com/BerriAI/litellm/issues/21560), priority/flex paygo pricing was added for Vertex AI. While this is correctly configured for newer models (like Gemini 3 and 3.1), it is missing for gemini-2.5-flash and gemini-2.5-flash-lite. This PR adds the missing priority/flex pricing configs for the Gemini 2.5 models.

see [vertex ai pricing](https://cloud.google.com/vertex-ai/generative-ai/pricing#priority_1)

### Motivation, pitch

I want to ensure accurate cost tracking for users still utilizing the Gemini 2.5 family. Currently, these models lack the priority/flex pricing logic that is already available for newer models, causing incorrect cost reporting.

### What part of LiteLLM is this about?

Proxy

### LiteLLM is hiring a founding backend engineer, are you interested in joining us and shipping to all our users?

No

### Twitter / LinkedIn details

_No response_