fix: Exclude closing fence markers from code block content

The process_fenced_code function was incorrectly including the closing
``` markers in the extracted code content. This happened because
text.rfind('\n') found the trailing newline after the closing fence
instead of the newline before it.

Fixed by trimming trailing newlines before finding the last newline
position, ensuring we correctly identify where the code content ends.

Added test_code_fence_content_excludes_closing_fence to prevent
regression.

Prompts:
- "code fences in dioxus are always showing the trailing ``` in the display mode"
- "you really don't know how to tdd do you. anyway, commit" (in response to review rejection)

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

Author: timabell

crates/markdown-neuraxis-engine/src/editing/snapshot.rs

@@ -479,8 +479,13 @@ fn process_fenced_code(
     // Extract code content between opening and closing fences
     let segments = if let Some(first_newline) = text.find('\n') {
         let content_start = node_range.start + first_newline + 1;
-        // Find last line (closing fence) by finding last newline before end
-        let last_newline = text.rfind('\n').unwrap_or(text.len());
+        // Find last line (closing fence) by finding last newline before the closing fence.
+        // We need to trim any trailing newline that comes AFTER the closing fence,
+        // otherwise rfind finds that trailing newline instead of the one before the fence.
+        let text_without_trailing = text.trim_end_matches('\n');
+        let last_newline = text_without_trailing
+            .rfind('\n')
+            .unwrap_or(text_without_trailing.len());
         // Content ends at the last newline (before closing fence line)
         let content_end = node_range.start + last_newline;
         if content_start < content_end {
@@ -1232,6 +1237,32 @@ mod tests {
         assert_eq!(block.kind, BlockKind::FencedCode { language: None });
     }
 
+    #[test]
+    fn test_code_fence_content_excludes_closing_fence() {
+        // The code fence content should NOT include the closing ``` markers
+        // Bug: when there's a trailing newline after closing fence, rfind('\n')
+        // finds that trailing newline instead of the one before the closing fence
+        let text = "```rust\nfn main() {}\n```\n";
+        let mut doc = Document::from_bytes(text.as_bytes()).unwrap();
+        doc.create_anchors_from_tree();
+        let snapshot = doc.snapshot();
+
+        assert_eq!(snapshot.blocks.len(), 1);
+        let block = &snapshot.blocks[0];
+
+        // Verify the segment content does not include the closing fence
+        assert_eq!(block.segments.len(), 1);
+        if let InlineNode::Text(content) = &block.segments[0].kind {
+            assert_eq!(content, "fn main() {}");
+            assert!(
+                !content.contains("```"),
+                "Content should not include closing fence, got: {content:?}"
+            );
+        } else {
+            panic!("Expected Text segment");
+        }
+    }
+
     #[test]
     fn test_snapshot_range_validity() {
         let text = "# Heading\n\n- Item 1\n- Item 2\n\nParagraph text";

crates/markdown-neuraxis-engine/src/editing/snapshots/complex_document.snap

@@ -40,7 +40,7 @@ Heading { level: 2 } [125..141]
     Text [128..140] "Code Example"
 FencedCode { language: Some("javascript") } [142..190]
   segments:
-    Text [156..189] "const x = 42;\nconsole.log(x);\n```"
+    Text [156..185] "const x = 42;\nconsole.log(x);"
 Heading { level: 2 } [191..201]
   segments:
     Text [194..200] "Quotes"

crates/markdown-neuraxis-engine/src/editing/snapshots/fenced_code_no_inlines.snap

@@ -4,7 +4,7 @@ expression: formatted
 ---
 FencedCode { language: None } [0..49]
   segments:
-    Text [4..48] "*not emphasis* **not strong** `not code`\n```"
+    Text [4..44] "*not emphasis* **not strong** `not code`"
 FencedCode { language: Some("rust") } [50..92]
   segments:
-    Text [58..91] "let x = \"[[not a wikilink]]\";\n```"
+    Text [58..87] "let x = \"[[not a wikilink]]\";"

crates/markdown-neuraxis-engine/src/editing/snapshots/fenced_code_no_lang.snap

@@ -4,4 +4,4 @@ expression: formatted
 ---
 FencedCode { language: None } [0..33]
   segments:
-    Text [4..32] "some code here\nmore code\n```"
+    Text [4..28] "some code here\nmore code"

crates/markdown-neuraxis-engine/src/editing/snapshots/fenced_code_with_lang.snap

@@ -4,4 +4,4 @@ expression: formatted
 ---
 FencedCode { language: Some("rust") } [0..57]
   segments:
-    Text [8..56] "fn main() {\n    println!(\"Hello, world!\");\n}\n```"
+    Text [8..52] "fn main() {\n    println!(\"Hello, world!\");\n}"

crates/markdown-neuraxis-engine/src/editing/snapshots/list_with_code_block.snap

@@ -12,7 +12,7 @@ List { ordered: false } [0..68]
         FencedCode { language: Some("python") } [20..53]
           root_range: 0..68
           segments:
-            Text [30..52] "  print(\"hello\")\n  ```"
+            Text [30..46] "  print(\"hello\")"
     ListItem { marker: "- " } [53..68]
       root_range: 0..68
       segments:

crates/markdown-neuraxis-engine/src/editing/snapshots/tilde_code_fence.snap

@@ -4,4 +4,4 @@ expression: formatted
 ---
 FencedCode { language: None } [0..25]
   segments:
-    Text [4..24] "code with tildes\n~~~"
+    Text [4..20] "code with tildes"