fix(file_editor): insert at EOF of file without trailing newline concatenates onto last line

[ksk2023]

HUMAN:

I manually reproduced the bug on the current main, applied the fix, and ran the file_editor test suite: my four new regression tests fail on main and all pass with the fix, with no new failures versus the baseline.

AGENT:

Issue Number

Fixes #4583

Why

FileEditor.insert silently corrupts a file when asked to insert after the last line of a file that does not end with a newline: the inserted content is concatenated onto the last line instead of starting on its own line. This is a real data-corruption path for agent-driven edits, and the bug was inherited from the original Anthropic computer-use-demo implementation this editor derives from.

Summary

In _execute_insert(), the file is read line-by-line and retained lines are collected up to insert_line. Python's line iterator omits the trailing \n from the last line when the file has no final newline, so new_lines[-1] is e.g. "c", and the new content is appended directly, producing cX\n.

The fix adds a guard right before appending the inserted lines:

if new_lines and not new_lines[-1].endswith("\n"):
    new_lines[-1] = new_lines[-1] + "\n"

Reproducer:

editor = FileEditor()
# file contains "a\nb\nc" (no trailing newline)
editor(command="insert", path=p, insert_line=3, new_str="X")
# Before: "a\nb\ncX\n"   After: "a\nb\nc\nX\n"

How to Test

Added 4 regression tests to tests/tools/file_editor/test_basic_operations.py:

• test_insert_at_eof_without_trailing_newline — single-line insert at EOF (fails on main, passes with fix)
• test_insert_multiple_lines_at_eof_without_trailing_newline — multi-line insert at EOF (fails on main, passes with fix)
• test_insert_at_eof_with_trailing_newline_unchanged — files already ending with \n keep existing behavior (no extra blank line)
• test_insert_in_middle_unaffected_by_eof_fix — middle-of-file inserts unchanged

Verified on main: 2 failed, 2 passed → with fix: 4 passed. The full file_editor suite shows no new failures compared to baseline (the 9 pre-existing failures on main under Windows — including test_insert_non_utf8_file, which fails identically without this change — are unrelated environment/encoding issues).

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[all-hands-bot]

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