refactor: simplify stylePreset merge logic in StabilityAiModel

[JHZLO]

Summary

This PR refactors the stylePreset configuration logic in StabilityAiImageOptions.Builder to remove redundant assignments and ensure consistent merging behavior.

Motivation

• Previously, stylePreset() could be called multiple times with potentially conflicting values.

• The new implementation consolidates this into a single line using:

  .stylePreset(ModelOptionsUtils.mergeOption(
      runtimeOptions.getStyle(),
      defaultOptions.getStylePreset()))

This change ensures that if a runtime option is provided, it takes precedence; otherwise, the default is used—matching the behavior of other portable image options such as model, width, and responseFormat.

Why use getStyle() and not getStylePreset() in runtime options?

• ImageOptions defines a getStyle() method, which is implemented in StabilityAiImageOptions as:

@Override
@JsonIgnore
public String getStyle() {
    return getStylePreset();
}

• The @JsonIgnore annotation ensures that this method is excluded from JSON serialization, so the style field does not accidentally appear in the outgoing payload.
• The actual JSON property used by the Stability API is style_preset, which is explicitly annotated:

@JsonProperty("style_preset")
private String stylePreset;

• This implementation keeps the merging logic interface-compliant via getStyle(), while ensuring that only style_preset is serialized in API requests.

Impact

• Improves clarity and predictability of the stylePreset merging process.
• Prevents unintended overrides or inconsistencies due to multiple stylePreset() calls.
• Ensures the correct field is serialized (style_preset) and avoids any accidental inclusion of an incorrect style field.

[sobychacko]

Merged via 2721c9e.