documents: edits read the caret off a CRLF split

The editor splits lines on LF only, so End can leave the caret
between CR and LF. The edit runner and table insert now settle the
selection before the CR; the line helpers assert they never see a
split instead of guessing.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/documents/document/tests/tools.rs

@@ -355,3 +355,33 @@ fn tidy_in_an_open_table_keeps_it_open(cx: &mut TestAppContext) {
         "| A    | B    |\n| ---- | ---- |\n| 1    | 2    |"
     );
 }
+
+const CRLF: &str = "Intro\r\nNext\r\n";
+
+/// A code block in CRLF text after `keys`.
+fn code_block_after(keys: &str, cx: &mut TestAppContext) -> String {
+    let (document, cx) = open(CRLF, MarkdownMode::Source, cx);
+    let editor = document.read_with(cx, |document, _| document.editor().clone());
+    let focus = editor.read_with(cx, gpui::Focusable::focus_handle);
+    cx.update(|window, cx| window.focus(&focus, cx));
+    cx.simulate_keystrokes(keys);
+    document.update(cx, |document, cx| {
+        document
+            .block(crate::documents::BlockCommand::CodeBlock, cx)
+            .expect("taken")
+    });
+    cx.run_until_parked();
+    text(&document, cx)
+}
+
+const FENCE_AFTER_INTRO: &str = "Intro\r\n\r\n```\r\n\r\n```\r\n\r\nNext\r\n";
+
+#[gpui::test]
+fn a_block_after_end_in_crlf_text_reads_the_line(cx: &mut TestAppContext) {
+    assert_eq!(code_block_after("end", cx), FENCE_AFTER_INTRO);
+}
+
+#[gpui::test]
+fn a_selection_from_inside_a_crlf_reads_the_line(cx: &mut TestAppContext) {
+    assert_eq!(code_block_after("end shift-right", cx), FENCE_AFTER_INTRO);
+}