Sticker generation was failing when using newer GPT image models
(gpt-image-1, gpt-image-1-mini, gpt-image-1.5). The issue occurred
because stickers requested 256x256 size, but these models only support
1024x1024, 1536x1024, 1024x1536, and auto.
To reproduce, send `!bai sticker Something` to an agent configured
with a GPT image model. The error was:
invalid_request_error: Invalid value: '256x256'. Supported values
are: '1024x1024', '1024x1536', '1536x1024', and 'auto'. (param: size)
(code: invalid_value)
The fix replaces the hardcoded 256x256 size override with a
`smallest_size_possible` flag, letting each provider determine the
appropriate sticker size based on the model being used.
The `openai_compat` provider still defaults to requesting 256x256 in all cases
(regardless of model name).
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