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/openai/utils.rs

@@ -1,8 +1,6 @@
-use async_openai::types::{
-    responses::{
-        EasyInputContent, EasyInputMessage, ImageDetail, InputContent, InputImageContent,
-        InputItem, InputParam, MessageType, Role,
-    },
+use async_openai::types::responses::{
+    EasyInputContent, EasyInputMessage, ImageDetail, InputContent, InputImageContent, InputItem,
+    InputParam, MessageType, Role,
 };
 
 use crate::conversation::llm::{
@@ -48,19 +46,3 @@ pub fn convert_llm_messages_to_openai_response_input(
 
     InputParam::Items(items)
 }
-
-pub(super) fn convert_string_to_enum<T>(value: &str) -> Result<T, String>
-where
-    T: serde::de::DeserializeOwned,
-{
-    // This is a hacky way to construct an enum from the string we have.
-    let enum_result: serde_json::Result<T> = serde_json::from_str(&format!("\"{}\"", value));
-    match enum_result {
-        Ok(enum_result) => Ok(enum_result),
-        Err(err) => {
-            tracing::debug!(?err, "Failed to parse into enum");
-
-            Err(format!("The value ({}) is not supported.", value))
-        }
-    }
-}