feat(open-sse): support dynamic image aspect ratios for antigravity image generation

[fasilu]

Problem

Gemini/Antigravity image generation requests lacked support for dynamic aspect ratios mapping from standard request dimensions.

Solution

• Intercept non-image model calls in the Antigravity image handler to map them to gemini-3.1-flash-image.
• Extract body.size parameters and resolve them to standard aspect ratios (e.g. 16:9 -> suffix -16x9).
• Append the aspect ratio suffix to the target model string to request the correct dimensions from the upstream provider.

Impact

• Enables full support for requesting different aspect ratios during image generation on Antigravity models.

Testing

• Verified that the test suite runs successfully and the changes introduce no new regressions.