[Bug]: Error parsing this PDF using "General" chunking method

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### RAGFlow workspace code commit ID

887651e

### RAGFlow image version

v0.19.1-2-gb4833f2d

### Other environment information

```Markdown
Ubuntu 24.04
Docker 28.3.0
RAGFlow 0.19.1
```

### Actual behavior

Whenever I try to parse this document [2025_7_00861.pdf](https://github.com/user-attachments/files/21148723/2025_7_00861.pdf) using the "General" chunking method I get an error and status "failed".

### Expected behavior

The document should be parsed without errors. The same document with the same configuration works fine in RAGFlow v0.18.

### Steps to reproduce

```Markdown
1. Upload the document to a datastore with "General" chunking method configured and chunk size 1024.
2. Parse the document.
3. Watch it fail.
```

### Additional information

Here is the log I got while trying to parse the document in v0.19.1:
[2025_7_00861.pdf.parsing_log.txt](https://github.com/user-attachments/files/21148773/2025_7_00861.pdf.parsing_log.txt)

By the way, the document does get parsed fine using "Paper" chunking method in v0.19.1.

Start by reproducing the failure with 2025_7_00861.pdf using the General chunking method at chunk size 1024, then inspect the attached parsing log and compare behavior with v0.18 and the Paper method in v0.19.1. Done means the document parses successfully with General chunking without regressing the Paper path.

Written by the indexing model from the issue text.