Fix completion auto-open position in notebook cells (#1398)

Fix regression in #1310

Bug: 


![auto-open-broke](https://github.com/microsoft/qsharp/assets/16928427/0c313bc4-69f1-4b19-a779-d9c483339123)


Fix:


![auto-open-fixed-2](https://github.com/microsoft/qsharp/assets/16928427/4029d801-1818-4399-9a51-1ffe34ce3f11)


![auto-open-fixed-1](https://github.com/microsoft/qsharp/assets/16928427/d8cacd35-1414-4b50-bf23-c50bc1eba8f7)

Author: minestarks

language_service/src/completion.rs

@@ -152,16 +152,26 @@ pub(crate) fn get_completions(
 }
 
 fn get_first_non_whitespace_in_source(compilation: &Compilation, package_offset: u32) -> u32 {
+    const QSHARP_MAGIC: &str = "//qsharp";
     let source = compilation
         .user_unit()
         .sources
         .find_by_offset(package_offset)
         .expect("source should exist in the user source map");
 
-    let first = source
-        .contents
-        .find(|c: char| !c.is_whitespace())
-        .unwrap_or(source.contents.len());
+    // Skip the //qsharp magic if it exists (notebook cells)
+    let start = if let Some(qsharp_magic_start) = source.contents.find(QSHARP_MAGIC) {
+        qsharp_magic_start + QSHARP_MAGIC.len()
+    } else {
+        0
+    };
+
+    let source_after_magic = &source.contents[start..];
+
+    let first = start
+        + source_after_magic
+            .find(|c: char| !c.is_whitespace())
+            .unwrap_or(source_after_magic.len());
 
     let first = u32::try_from(first).expect("source length should fit into u32");
 

language_service/src/completion/tests.rs

@@ -1038,8 +1038,20 @@ fn notebook_block() {
 fn notebook_auto_open_start_of_cell_empty() {
     check_notebook(
         &[
-            ("cell1", "namespace Foo { operation Bar() : Unit {} }"),
-            ("cell2", "↘"),
+            (
+                "cell1",
+                indoc! {"
+                    //qsharp
+                    namespace Foo { operation Bar() : Unit {} }"
+                },
+            ),
+            (
+                "cell2",
+                indoc! {"
+                    //qsharp
+                    ↘"
+                },
+            ),
         ],
         &["Fake"],
         &expect![[r#"
@@ -1060,11 +1072,11 @@ fn notebook_auto_open_start_of_cell_empty() {
                                     new_text: "open FakeStdLib;\n",
                                     range: Range {
                                         start: Position {
-                                            line: 0,
+                                            line: 1,
                                             column: 0,
                                         },
                                         end: Position {
-                                            line: 0,
+                                            line: 1,
                                             column: 0,
                                         },
                                     },
@@ -1082,8 +1094,21 @@ fn notebook_auto_open_start_of_cell_empty() {
 fn notebook_auto_open_start_of_cell() {
     check_notebook(
         &[
-            ("cell1", "namespace Foo { operation Bar() : Unit {} }"),
-            ("cell2", r#"   Message("hi") ↘"#),
+            (
+                "cell1",
+                indoc! {"
+                    //qsharp
+                    namespace Foo { operation Bar() : Unit {} }"
+                },
+            ),
+            (
+                "cell2",
+                indoc! {r#"
+                    //qsharp
+                    Message("hi")
+                    ↘"#
+                },
+            ),
         ],
         &["Fake"],
         &expect![[r#"
@@ -1101,15 +1126,15 @@ fn notebook_auto_open_start_of_cell() {
                         additional_text_edits: Some(
                             [
                                 TextEdit {
-                                    new_text: "open FakeStdLib;\n   ",
+                                    new_text: "open FakeStdLib;\n",
                                     range: Range {
                                         start: Position {
-                                            line: 0,
-                                            column: 3,
+                                            line: 1,
+                                            column: 0,
                                         },
                                         end: Position {
-                                            line: 0,
-                                            column: 3,
+                                            line: 1,
+                                            column: 0,
                                         },
                                     },
                                 },