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/agent/provider/entity/image.rs

@@ -2,7 +2,7 @@ use mxlink::mime;
 
 #[derive(Default)]
 pub struct ImageGenerationParams {
-    pub size_override: Option<String>,
+    pub smallest_size_possible: bool,
 
     pub cheaper_model_switching_allowed: bool,
 
@@ -10,8 +10,8 @@ pub struct ImageGenerationParams {
 }
 
 impl ImageGenerationParams {
-    pub fn with_size_override(mut self, value: Option<String>) -> Self {
-        self.size_override = value;
+    pub fn with_smallest_size_possible(mut self, value: bool) -> Self {
+        self.smallest_size_possible = value;
         self
     }