Fix: improve PDF text type detection by expanding regex content (#11432)

- Add whitespace validation to the PDF English text checking regex
- Reduce false negatives in English PDF content recognition

### What problem does this PR solve?

The core idea is to **expand the regex content used for English text
detection** so it can accommodate more valid characters commonly found
in English PDFs. The modifications include:

- Adding support for **space** in the regex.
- Ensuring the update does not reduce existing detection accuracy.

### Type of change

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

deepdoc/parser/pdf_parser.py

@@ -1091,7 +1091,7 @@ class RAGFlowPdfParser:
 
         logging.debug("Images converted.")
         self.is_english = [
-            re.search(r"[a-zA-Z0-9,/¸;:'\[\]\(\)!@#$%^&*\"?<>._-]{30,}", "".join(random.choices([c["text"] for c in self.page_chars[i]], k=min(100, len(self.page_chars[i])))))
+            re.search(r"[ a-zA-Z0-9,/¸;:'\[\]\(\)!@#$%^&*\"?<>._-]{30,}", "".join(random.choices([c["text"] for c in self.page_chars[i]], k=min(100, len(self.page_chars[i])))))
             for i in range(len(self.page_chars))
         ]
         if sum([1 if e else 0 for e in self.is_english]) > len(self.page_images) / 2:
@@ -1148,7 +1148,7 @@ class RAGFlowPdfParser:
 
         if not self.is_english and not any([c for c in self.page_chars]) and self.boxes:
             bxes = [b for bxs in self.boxes for b in bxs]
-            self.is_english = re.search(r"[\na-zA-Z0-9,/¸;:'\[\]\(\)!@#$%^&*\"?<>._-]{30,}", "".join([b["text"] for b in random.choices(bxes, k=min(30, len(bxes)))]))
+            self.is_english = re.search(r"[ \na-zA-Z0-9,/¸;:'\[\]\(\)!@#$%^&*\"?<>._-]{30,}", "".join([b["text"] for b in random.choices(bxes, k=min(30, len(bxes)))]))
 
         logging.debug(f"Is it English: {self.is_english}")