Take the line break from the line above when the last line has none

The open goes in front of a line, and the last line of a file often ends
without a line break. The file's own line ending is then on the line above,
so a file with LF endings no longer gets CRLF from the host.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: xperiandri

vsintegration/src/FSharp.Editor/Common/RoslynHelpers.fs

@@ -247,9 +247,14 @@ module internal OpenDeclarationHelper =
         let pos = ParsedInput.AdjustInsertionPoint getLineStr ctx
         let line = sourceText.Lines[min (Line.toZ pos.Line) (sourceText.Lines.Count - 1)]
 
-        // Follow the line endings the file itself uses rather than assuming the host's.
+        let lineBreakOf (line: TextLine) =
+            sourceText.ToString(TextSpan.FromBounds(line.End, line.EndIncludingLineBreak))
+
+        // Follow the line endings the file itself uses rather than assuming the host's. Only the last
+        // line can lack a line break, and every line above it has one.
         let lineBreak =
-            match line.Text.ToString(TextSpan(line.End, line.EndIncludingLineBreak - line.End)) with
+            match lineBreakOf line with
+            | "" when line.LineNumber > 0 -> lineBreakOf sourceText.Lines[line.LineNumber - 1]
             | "" -> Environment.NewLine
             | breakChars -> breakChars
 

vsintegration/tests/FSharp.Editor.Tests/CodeFixes/AddOpenOnTopOnTests.fs

@@ -200,6 +200,21 @@ Console.WriteLine 42
 
     Assert.Equal(expected, actual)
 
+[<Fact>]
+let ``Fixes FS0039 for missing opens - the open goes before a last line with no line break`` () =
+    let code = "module Module1\nConsole.WriteLine 42"
+
+    let expected =
+        Some
+            {
+                Message = "open System"
+                FixedCode = "module Module1\n\nopen System\n\nConsole.WriteLine 42"
+            }
+
+    let actual = codeFix |> tryFix code Auto
+
+    Assert.Equal(expected, actual)
+
 [<Fact>]
 let ``Fixes FS0039 for missing opens - namespace without a blank line`` () =
     let code =