fix(shell): use byte length for preview truncation

[SpencerJung]

Fixes #29291. The preview() function used text.length (UTF-16 code units) to check if output exceeds the metadata limit, but truncation should be based on byte length. This caused incorrect truncation for multi-byte characters like CJK, emoji, etc. Additionally, text.slice(-MAX_METADATA_LENGTH) could split a surrogate pair. Use Buffer.byteLength for the check and Buffer-based truncation that respects UTF-8 continuation bytes, matching the existing tail() function pattern.

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The following comment was made by an LLM, it may be inaccurate:

Potential duplicates found:

1. PR fix(shell): use byte length for preview truncation (fixes #29291) #29372 - fix(shell): use byte length for preview truncation (fixes #29291) - This appears to be addressing the same issue (shell tool preview truncation uses char length instead of byte length #29291) with the same fix approach.

2. PR fix(shell): truncate metadata preview by bytes #29297 - fix(shell): truncate metadata preview by bytes - Related fix for metadata preview truncation using bytes.

3. PR fix(shell): preview Unicode truncation uses byte length instead of char length #29288 - fix(shell): preview Unicode truncation uses byte length instead of char length - Addresses the same core issue of using byte length instead of character length for preview truncation.

These PRs are all targeting the same bug in shell preview truncation with multi-byte characters. PR #29372 appears to be the most direct duplicate as it fixes the exact same issue (#29291) with the same title.

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