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/controller/image/generation.rs

@@ -12,9 +12,6 @@ use crate::strings;
 use crate::utils::mime::get_file_extension;
 use crate::{Bot, entity::MessageContext};
 
-// We may make this configurable (per room, etc.) in the future, but for now it's hardcoded.
-const STICKER_SIZE: &str = "256x256";
-
 pub async fn handle_image(
     bot: &Bot,
     matrix_link: MatrixLink,
@@ -177,7 +174,7 @@ pub async fn handle_sticker(
     );
 
     let params = ImageGenerationParams::default()
-        .with_size_override(Some(STICKER_SIZE.to_owned()))
+        .with_smallest_size_possible(true)
         .with_cheaper_model_switching_allowed(true)
         .with_cheaper_quality_switching_allowed(true);