Fix sticker generation for newer GPT image models

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).

src/strings/usage.rs

@@ -64,7 +64,7 @@ To create a sticker, send a command like `%command_prefix% sticker A huge bowl o
 
 The difference from **creating images** is that the bot will:
 
-- create a smaller-resolution image (`256x256`) - smaller/quicker, but still good enough for a sticker
+- create a smaller-resolution image (as small as the model allows) - smaller/quicker, but still good enough for a sticker
 - potentially switch to a different (cheaper or otherwise more suitable) model, if available
 - post the image directly to the room (as a reply to your message), without starting a threaded conversation
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