## Why
Token-budget truncation uses byte counts to estimate tokens, but previously scanned the entire string to count characters, including the discarded middle.
## What changed
Use UTF-8 boundary lookups to select the retained prefix and suffix for token-budget truncation. Skip character counting for a zero-token budget and defer it to paths that need character counts.
## Testing
Add regression coverage for multibyte characters crossing both retained boundaries, checking the truncated output and original token estimate.
GitOrigin-RevId: 37fee0c8d4ede8e74b491d3d641b0ea0e9003dd4