fix(exec): truncate output on utf8 boundaries

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53116.

truncate() previously sliced output with &s[..max_len], which can panic when max_len falls inside a multi-byte UTF-8 scalar. This changes truncation to pick the nearest previous character boundary before slicing, preserving the existing byte-length cap behavior without panicking on Unicode output.

Validation:

• cargo +1.90.0 fmt --package cortex-exec --check
• git diff --check
• isolated rustc +1.90.0 --test harness covering ASCII, Japanese text, and emoji truncation

Note: full cargo +1.90.0 test -p cortex-exec ... is currently blocked on this Windows machine by a pre-existing workspace dependency build failure in aws-lc-sys before cortex-exec tests run.

[greptile-apps]

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