Fix kb isolation infinity conn (#9913)

### What problem does this PR solve?

This PR fixes a critical bug in the knowledge base isolation feature
where chat responses were referencing documents from incorrect knowledge
bases. The issue was in the `infinity_conn.py` file where the
`equivalent_condition_to_str()` function was incorrectly skipping
`kb_id` filtering, causing documents from unintended knowledge bases to
be included in search results.

### Type of change

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

---------

Co-authored-by: Syed Shahmeer Ali <ashahmeer73@gmail.com>
Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

Author: 3 people

rag/utils/infinity_conn.py

@@ -67,7 +67,7 @@ def exists(cln):
 
     cond = list()
     for k, v in condition.items():
-        if not isinstance(k, str) or k in ["kb_id"] or not v:
+        if not isinstance(k, str) or not v:
             continue
         if field_keyword(k):
             if isinstance(v, list):