Fix JSX expression regex to handle nested braces

Replace simple regex pattern that failed on nested braces with
existing BRACE_PATTERN that properly handles up to 3 levels of
nesting. This fixes issues with expressions like {obj || {default: true}}
and {items.map(x => ({a: x}))} where stray closing braces would remain
in the output.

Author: cursoragent

app/api/markdown/[[...slug]]/route.ts

@@ -35,7 +35,7 @@ const JSX_WITH_CHILDREN_REGEX = new RegExp(
   "g"
 );
 const CODE_BLOCK_REGEX = /```[\s\S]*?```/g;
-const JSX_EXPRESSION_REGEX = /\{[^}]+\}/g;
+const JSX_EXPRESSION_REGEX = new RegExp(BRACE_PATTERN, "g");
 const EXCESSIVE_NEWLINES_REGEX = /\n{3,}/g;
 const CODE_BLOCK_PLACEHOLDER_REGEX = /__CODE_BLOCK_(\d+)__/g;