Fix asset_type in creative manifest prompts

[bokelley]

Background

The ADCP spec no longer requires asset_type in creative manifest assets. This change corrects AI prompts that were incorrectly including it.

Changes

• Modified AI prompts in src/creative_agent/server.py (lines 669-671 and 692-693) to remove the asset_type field from example asset structures.
  • Before: {"asset_type": "image", "url": "GENERATED_IMAGE_PLACEHOLDER", ...}
  • After: {"url": "GENERATED_IMAGE_PLACEHOLDER", ...}

Testing

• Manually preview a creative that requires image, text, and URL assets.
• Verify that no asset_type is present in the generated manifest.