SDK-5993: make vertical image corner radius and border configurable

[reshab-code]

Summary

Made the vertical image corner radius, border color, and border width configurable via push payload keys (pt_img_corner_radius, pt_img_border_clr, pt_img_border_width). This mirrors the existing button border configuration pattern and allows templates to control image styling declaratively.

• Added three optional payload keys to VerticalImageProperties
• Moved hardcoded cornerRadius = 8.0 to configurable setting with 8pt default (preserves existing behavior)
• Border only renders when pt_img_border_clr is provided; pt_img_border_width defaults to 1.0pt
• Reuses existing FlexibleDouble decoder and UIColor(hex:) color parser

Test plan

Send vertical image push notifications with different payload configurations:

• No image keys → image shows 8pt rounded corners, no border (regression)
• pt_img_corner_radius: 20 → visibly rounder corners
• pt_img_border_clr: "#FF0000", pt_img_border_width: 3 → red 3pt border
• pt_img_corner_radius: "16" (string value) → confirm FlexibleDouble parsing
• All three keys combined → full styling control

[shubhambjadhavar]

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