fix: base64 encode Google inline image bytes

[aspire488]

Problem or current behavior

Result or new behavior

Compatibility and migration

Validation

Agent review

Summary

Fixes conversion of Google GenAI inline image data in
messages_from_google().

Blob.data contains raw bytes, but the current conversion decodes those
bytes as UTF-8 before passing them to as_data_uri(). Real image bytes can
therefore raise UnicodeDecodeError, and UTF-8-decodable bytes can produce
invalid base64 payloads.

Changes

• Base64-encode raw inline_data.data bytes before constructing the data URI.
• Check explicitly for None rather than truthiness.
• Add a regression test using binary PNG bytes.

This mirrors the existing Google provider conversion path.