feat(registry, executor, util): add support for gemini-2.5-flash-image-preview and improve aspect ratio handling

- Introduced `gemini-2.5-flash-image-preview` model to the registry with updated definitions.
- Enhanced Gemini CLI and API executors to handle image aspect ratio adjustments for the new model.
- Added utility function to create base64 white image placeholders based on aspect ratio configurations.

internal/util/image.go

@@ -0,0 +1,59 @@
+package util
+
+import (
+	"bytes"
+	"encoding/base64"
+	"image"
+	"image/draw"
+	"image/png"
+)
+
+func CreateWhiteImageBase64(aspectRatio string) (string, error) {
+	width := 1024
+	height := 1024
+
+	switch aspectRatio {
+	case "1:1":
+		width = 1024
+		height = 1024
+	case "2:3":
+		width = 832
+		height = 1248
+	case "3:2":
+		width = 1248
+		height = 832
+	case "3:4":
+		width = 864
+		height = 1184
+	case "4:3":
+		width = 1184
+		height = 864
+	case "4:5":
+		width = 896
+		height = 1152
+	case "5:4":
+		width = 1152
+		height = 896
+	case "9:16":
+		width = 768
+		height = 1344
+	case "16:9":
+		width = 1344
+		height = 768
+	case "21:9":
+		width = 1536
+		height = 672
+	}
+
+	img := image.NewRGBA(image.Rect(0, 0, width, height))
+	draw.Draw(img, img.Bounds(), image.White, image.Point{}, draw.Src)
+
+	var buf bytes.Buffer
+
+	if err := png.Encode(&buf, img); err != nil {
+		return "", err
+	}
+
+	base64String := base64.StdEncoding.EncodeToString(buf.Bytes())
+	return base64String, nil
+}