Fix invalid number of space characters inserted for tab (#27336)

Closes #25941 

Release Notes:

- Corrected SoftTab indentation handling for lines with mixed spaces and
tabs across .go files and other file types.
- Renamed the editor test `test_tab_with_mixed_whitespace` to
`test_tab_with_mixed_whitespace_rust` as it only tested this behavior
for Rust buffers, which have auto-indentation support. This change
clarifies that the test does not cover default files without
language-specific features.
- Added a new editor test `test_tab_with_mixed_whitespace_txt` to ensure
proper coverage for files with no associated language.

While investigating the issue — initially thought to be Go-related — I
discovered that the underlying problem was how soft tabs were calculated
in `Editor::tab`, given that the problem could also be observed on
`.txt` files

The correct soft tab indentation is now determined by treating all `\t`
characters before the cursor (on the same row) as new indentation
levels, resetting the remainder counter accordingly.


https://github.com/user-attachments/assets/78192e98-2b81-43cb-ae6f-7c48cd17d168

crates/editor/src/editor.rs

@@ -8209,12 +8209,18 @@ impl Editor {
                 IndentSize::tab()
             } else {
                 let tab_size = settings.tab_size.get();
-                let char_column = snapshot
+                let indent_remainder = snapshot
                     .text_for_range(Point::new(cursor.row, 0)..cursor)
                     .flat_map(str::chars)
-                    .count()
-                    + row_delta as usize;
-                let chars_to_next_tab_stop = tab_size - (char_column as u32 % tab_size);
+                    .fold(row_delta % tab_size, |counter: u32, c| {
+                        if c == '\t' {
+                            0
+                        } else {
+                            (counter + 1) % tab_size
+                        }
+                    });
+
+                let chars_to_next_tab_stop = tab_size - indent_remainder;
                 IndentSize::spaces(chars_to_next_tab_stop)
             };
             selection.start = Point::new(cursor.row, cursor.column + row_delta + tab_size.len);