Teknium 5f84c9144a
fix(file-tools): handle UTF-8 BOM in read_file / write_file / patch (#35278)
Some Windows editors prepend an invisible UTF-8 BOM (U+FEFF) to text
files. We had no awareness of it, so: read_file surfaced a phantom
U+FEFF as the first character; patch matches against the true first
line could miss; and a write/patch round-trip silently stripped the
marker, changing the file's byte signature.

Now:
- read_file / read_file_raw strip a single leading BOM so the model
  never sees it (only on the first chunk — the marker lives at byte 0).
- patch_replace strips the BOM before fuzzy-matching (so an exact
  first-line match works) and its post-write verification compares
  BOM-stripped content.
- write_file restores the BOM when the original file had one and the
  new content doesn't, mirroring the existing line-ending preservation
  (detect on disk via a cheap `head -c 3` probe or reuse pre_content,
  re-prepend across the edit). Guards against double-BOM.

Mid-content U+FEFF is left alone (it's data there, not a file marker).

Tests: TestBomHandling (real LocalEnvironment) — read-strips, raw-read
strips, write preserves, no-BOM-when-original-had-none, no-double-BOM,
patch round-trip preserves, patch matches first line through a BOM,
plus helper unit tests. 208 file-tool tests green.
2026-05-30 06:25:50 -07:00
..