feat(ai): add missing HARM_CATEGORY_IMAGE_* enum types

[Lyokone]

Description

Add the four HARM_CATEGORY_IMAGE_* values to HarmCategory so safety ratings returned by image generation models parse correctly instead of throwing FormatException: Unhandled HarmCategory format. Unknown categories now fall back to HarmCategory.unknown for forward compatibility.

Related Issues

• Fixes [firebase_ai]: Support for HARM_CATEGORY_IMAGE_SEXUALLY_EXPLICIT and HARM_CATEGORY_IMAGE_DANGEROUS_CONTENT in HarmCategory enum #17472

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

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