Closes#1441. When `@`-mentioning a file larger than the 128 KB
`MAX_MENTION_FILE_BYTES` ceiling, the truncator clipped the buffer
to exactly the cap — which on CJK / emoji content frequently
landed mid-codepoint and left a stray U+FFFD replacement char at
the cut point.
The fix uses `str::from_utf8(...).error_len()` to distinguish the
two ways a truncated UTF-8 buffer can fail:
- `error_len() == None` means the failure is an incomplete tail
sequence — exactly the boundary case we want to handle. Round
`buffer.truncate()` down to `valid_up_to()` so the trailing
bytes are dropped cleanly.
- `error_len() == Some(_)` means the file genuinely contains
invalid UTF-8 bytes (not at the truncation boundary). Leave
the buffer intact so the subsequent `from_utf8(&buffer)` call
surfaces the canonical "file is not UTF-8" error rather than
silently dropping the invalid bytes.
Collapsed the if-let-then-if pattern to `if let Err(e) = ... &&
e.error_len().is_none()` to satisfy the workspace's
`collapsible_if` clippy gate.
Harvested from PR #1495 by @CrepuscularIRIS
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Hunter Bown
6f70a2832e