fix(issues): Decode text attachment previews from raw bytes (#122677)

Text attachment previews currently go through response.text(), which
always decodes as UTF-8 and garbles UTF-16 crash logs.

Fetches preview bytes directly and handles UTF-16 byte order marks
locally, keeping the shared API client untouched.

Fixes #91336
Fixes #122204

---------

Co-authored-by: OpenAI Codex <noreply@openai.com>