core(cua): Google function-response image handling hardcodes PNG mimeType

[BABTUNA]

Why

GoogleCUAClient currently hardcodes mimeType: "image/png" and strips only a PNG data URL prefix when building function-response image parts.

If screenshot input is JPEG (or any non-PNG data URL), we send mismatched metadata and may pass the full data URL instead of raw base64 payload.

Current behavior

In packages/core/lib/v3/agent/GoogleCUAClient.ts:

• function responses always set inlineData.mimeType to image/png
• payload extraction strips only ^data:image/png;base64,
• captureScreenshot() normalizes raw provider input to PNG data URLs

Proposed change

• Parse MIME + base64 payload from screenshot data URLs
• Use parsed MIME in function-response inlineData.mimeType
• Fallback to image/png for raw/non-image data URLs to preserve compatibility
• Add unit tests covering JPEG parsing and PNG fallback

Suggested files

• packages/core/lib/v3/agent/GoogleCUAClient.ts
• packages/core/tests/unit/google-cua-client.test.ts

Acceptance criteria

• function-response inlineData.mimeType matches screenshot data URL MIME (for image data URLs)
• base64 payload excludes data URL prefix for PNG/JPEG
• existing raw base64 behavior still works via PNG fallback
• unit tests cover MIME parsing and fallback