fix(markdown): respect fenced block closers

packages/shared/src/validators/text.test.ts

@@ -62,6 +62,11 @@ describe("normalizeMidLineHeadings", () => {
     expect(normalizeMidLineHeadings(body)).toBe(body);
   });
 
+  it("does not close a fence marker that has trailing content", () => {
+    const body = "```markdown\n``` still code\ncode ## comment\n```";
+    expect(normalizeMidLineHeadings(body)).toBe(body);
+  });
+
   it("is a no-op when there is no `#` at all", () => {
     const body = "Just some prose with no markers.";
     expect(normalizeMidLineHeadings(body)).toBe(body);

packages/shared/src/validators/text.ts

@@ -7,7 +7,7 @@ export function normalizeEscapedLineBreaks(value: string): string {
     .replace(/\\r/g, "\n");
 }
 
-const FENCE_LINE = /^(\s{0,3})(```+|~~~+)/;
+const FENCE_LINE = /^(\s{0,3})(```+|~~~+)(.*)$/;
 // A mid-line ATX heading marker: 1–6 `#`, a space, then heading text. The
 // capture keeps the char immediately before the run so we can tell a real
 // (collapsed) heading from an inline `#` token like `C#`/`F#`.
@@ -53,7 +53,11 @@ export function normalizeMidLineHeadings(value: string): string {
         inFence = true;
         fenceChar = marker[0]!;
         fenceLength = marker.length;
-      } else if (marker[0] === fenceChar && marker.length >= fenceLength) {
+      } else if (
+        marker[0] === fenceChar
+        && marker.length >= fenceLength
+        && fenceMatch[3]!.trim().length === 0
+      ) {
         inFence = false;
         fenceChar = "";
         fenceLength = 0;