Fix: Unnecessary truncation in markdown parser (#7972)

### What problem does this PR solve?

Fix unnecessary truncation in markdown parser. So that markdown can work
perfectly like
[this](https://github.com/infiniflow/ragflow/issues/7824#issuecomment-2921312576)
in #7824, supporting multiple special delimiters.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/nlp/__init__.py

@@ -545,7 +545,7 @@ def naive_merge(sections, chunk_token_num=128, delimiter="\n。；！？"):
             add_chunk(sub_sec, pos)
 
     return cks
-    
+
 
 def naive_merge_with_images(texts, images, chunk_token_num=128, delimiter="\n。；！？"):
     if not texts or len(texts) != len(images):
@@ -676,6 +676,8 @@ def get_delimiters(delimiters: str):
         s = t
     if s < len(delimiters):
         dels.extend(list(delimiters[s:]))
+
+    dels.sort(key=lambda x: -len(x))
     dels = [re.escape(d) for d in dels if d]
     dels = [d for d in dels if d]
     dels_pattern = "|".join(dels)