* fix(openai): use truncateUtf16Safe for image gen log value truncation
One .slice(0, N) truncation site in the OpenAI image generation
provider may cut UTF-16 surrogate pairs in half when the log value
contains multi-byte characters such as emoji. Replace it with
truncateUtf16Safe to keep the truncated output valid Unicode.
* fix(openai): keep image auth logs UTF-16 safe
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Co-authored-by: Peter Steinberger <steipete@gmail.com>