fix(fs_write): suppress spurious trailing-newline diff lines

[sandikodev]

Issue

Closes #923

Problem

When fs_write modifies a file, the diff shown to the user always has a spurious last line:

- 92    : fi
+     92: fi

The content is identical — the only difference is a trailing newline. This is a cosmetic artefact caused by write_to_file always appending \n to file content, while the diff compares the raw strings before normalization.

Fix

Strip at most one trailing newline from both sides before passing to TextDiff::from_lines:

// Before
let diff = similar::TextDiff::from_lines(&old_str.content, &new_str.content);

// After — strip_suffix removes at most one \n, preserving intentional multi-newline differences
let old_content = old_str.content.strip_suffix('\n').unwrap_or(&old_str.content);
let new_content = new_str.content.strip_suffix('\n').unwrap_or(&new_str.content);
let diff = similar::TextDiff::from_lines(old_content, new_content);

strip_suffix (not trim_end_matches) is used deliberately — it removes exactly one trailing newline so that intentional multi-newline differences (e.g. old has 1 trailing newline, new has 3) are still shown.

Testing

# Run the three targeted tests
cargo test -p chat_cli test_print_diff

# Run the full test suite to check for regressions
cargo test -p chat_cli

Three tests cover all scenarios:

1. test_print_diff_trailing_newline_only_change_is_not_shown — the bug case: trailing-newline-only diff produces zero deletions and zero insertions
2. test_print_diff_real_change_is_shown — regression guard: a genuine content change still appears (uses insta::assert_snapshot! for the exact formatted output)
3. test_print_diff_multiple_trailing_newlines_difference_is_shown — edge case: intentional extra newlines are still shown as insertions

Tests use strip_ansi_escapes (already in the workspace) to strip terminal color codes before asserting.

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[sandikodev]

Update (amended commit): The fix was originally using trim_end_matches('\n') which strips all trailing newlines, silently hiding intentional multi-newline differences. Changed to strip_suffix('\n') which removes at most one — enough to eliminate the write_to_file artefact without swallowing real content changes.

Three tests were added covering all scenarios:

1. Trailing-newline-only difference → not shown (the bug case)
2. Real content change → still shown (regression guard)
3. Multiple trailing newlines difference → still shown (edge case guard)

Tests use strip_ansi_escapes (already in the workspace) to strip terminal color codes before asserting, and insta::assert_snapshot! for the formatted diff output.