fix(core): replace custom binary detection with isbinaryfile to correctly handle UTF-8 (U+FFFD)

[Anjaligarhwal]

Summary

Fixes #24547. Replaces the hand-rolled UTF-8 byte heuristic in isBinaryFile() with a delegation to the isbinaryfile npm package, so source files that legitimately contain the Unicode replacement character U+FFFD (EF BF BD in UTF-8) are no longer misclassified as binary.

Details

• Adds isbinaryfile@^5.0.7 as a packages/core dependency (Node ≥20 compatible, zero transitive deps, MIT-licensed, same library used by ESLint / Turborepo).
• isBinaryFile() in packages/core/src/utils/fileUtils.ts becomes a thin wrapper over isbinaryfile's async API. Public signature is unchanged, so no call sites need updating.
• The library handles Unicode BOMs (UTF-8/16/32/GB-18030), PDF magic, NULL bytes, valid UTF-8 multibyte sequences, and an invalid-byte ratio (overlong encodings, post-U+10FFFF, DEL character).
• detectBOM() is kept in place because readFileWithEncoding and isEmpty still use it for BOM-aware decoding.
• Error semantics preserved: I/O errors (ENOENT, path-is-directory, etc.) still resolve to false with a debugLogger.warn, matching previous behavior for callers downstream.

Related Issues

• Closes 'U+FFFD' character in file broken by replace tool #24547

How to Validate

1. cd packages/core && npm run typecheck && npm run lint
2. cd packages/core && npx vitest run src/utils/fileUtils.test.ts — 16 isBinaryFile tests pass (5 existing + 3 new regression tests for U+FFFD, mixed UTF-8, and invalid high-byte sequences, plus 8 BOM tests).
3. cd packages/core && npx vitest run src/tools/read-file.test.ts src/tools/read-many-files.test.ts — all downstream consumers clean.
4. Manual: create a file containing '\uFFFD', run read_file / replace against it — content preserved, no hexdump corruption.

Pre-Merge Checklist

• npm run typecheck passes
• npm run lint passes
• npx prettier --check passes
• Unit tests pass (including 3 new regression guards)
• No behavior change for callers; function signature identical

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug where files containing the Unicode replacement character (U+FFFD) were erroneously classified as binary, leading to incorrect file processing. By replacing the existing custom binary detection logic with the robust isbinaryfile npm package, the system now accurately distinguishes between text and binary files, ensuring proper handling of diverse character encodings and preventing data corruption or misinterpretation.

Highlights

• Dependency Update: Integrated the isbinaryfile npm package (^5.0.7) into packages/core to improve binary file detection.
• Improved Binary Detection: Replaced the custom isBinaryFile() heuristic with the isbinaryfile package, resolving false positives for files containing the Unicode replacement character (U+FFFD).
• Enhanced Test Coverage: Added three new regression tests to fileUtils.test.ts to specifically validate correct handling of U+FFFD, mixed UTF-8 content, and dense invalid UTF-8 byte sequences.

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[gemini-code-assist]

Code Review

This pull request replaces the custom binary file detection logic in packages/core/src/utils/fileUtils.ts with the isbinaryfile package. This change addresses issue #24547, where files containing the Unicode replacement character (U+FFFD) were incorrectly identified as binary. New test cases have been added to verify correct handling of U+FFFD, mixed CJK/emoji content, and actual binary data. I have no feedback to provide.

[Anjaligarhwal]

Thanks for the review @scidomino, all four items addressed in 126ea0e -Trimmed JSDoc on isBinaryFile() to two sentences, removed inline comments from all three new test cases
PTAL when you get a chance

[scidomino]

Note: don't push any updates now or we will have to restart the CI checks

[Anjaligarhwal]

Understood, i won't push anything thanks for handling this!