fix(chunkers): exclude lookbehind from named-group rewrite

Tighten NAMED_GROUP_PREFIX with negative lookahead so patterns like
(?<=<tag>) are not misidentified as named capture groups.

Author: waleedlatif1

apps/sim/lib/chunkers/regex-chunker.test.ts

@@ -227,6 +227,21 @@ describe('RegexChunker', () => {
       }
     )
 
+    it.concurrent('should preserve lookbehind whose body contains a > character', async () => {
+      const chunker = new RegexChunker({
+        pattern: '(?<=</section>)',
+        chunkSize: 1024,
+        strictBoundaries: true,
+      })
+      const text = '<section>one</section><section>two</section><section>three</section>'
+      const chunks = await chunker.chunk(text)
+
+      expect(chunks).toHaveLength(3)
+      expect(chunks[0].text).toBe('<section>one</section>')
+      expect(chunks[1].text).toBe('<section>two</section>')
+      expect(chunks[2].text).toBe('<section>three</section>')
+    })
+
     it.concurrent('should leave non-capturing groups and lookarounds intact', async () => {
       const chunker = new RegexChunker({
         pattern: '(?=\\n\\s*\\{\\s*"id"\\s*:)',

apps/sim/lib/chunkers/regex-chunker.ts

@@ -15,7 +15,7 @@ const logger = createLogger('RegexChunker')
 
 const MAX_PATTERN_LENGTH = 500
 
-const NAMED_GROUP_PREFIX = /^\(\?<[^>]+>/
+const NAMED_GROUP_PREFIX = /^\(\?<(?![=!])[^>]+>/
 
 /**
  * Converts unescaped capturing groups `(...)` and named capturing groups