[Bug]: Google Image models don't respect output_format

[jaimeiniesta]

Provider (if applicable)

google

Model (if applicable)

google:gemini-2.5-flash-image

Bug Description

The output_format parameter, which can be png, jpeg or webp, doesn't have any effect on Google image models, and always returns PNG.

This works fine with OpenAI models.

Reproduction Code

Here's a gist with a Livebook

https://gist.github.com/jaimeiniesta/a1061b605b65356d55c300b88a73d1ed

Expected Behavior

Return PNG, JPEG or WEBP as requested.

Actual Behavior

It always returns PNG.

Environment

• ReqLLM 1.5.0 (also fails on 1.3.0)
• Elixir 1.19.2
• Erlang/OTP 28