[BUG]: Missing cost estimate for gemini-3.1-flash-image-preview model

### Tracer Version(s)

4.6.2

### Python Version(s)

Python 3.14.3

### Pip Version(s)

uv 0.10.10

### Bug Report

In Datadog traces LLObs, , while I am getting the correct input and output tokens, the estimated cost is showing as "n/a".

### Reproduction Code

_No response_

### Error Logs

_No response_

### Libraries in Use

_No response_

### Operating System

_No response_