refactor: streamline document upload limits and enhance handling of mentioned documents

- Updated maximum file size limit to 500 MB per file.
- Removed restrictions on the number of files per upload and total upload size.
- Enhanced handling of user-mentioning documents in the knowledge base search middleware.
- Improved document reading and processing logic to accommodate new features and optimizations.

surfsense_backend/tests/unit/middleware/test_knowledge_search.py

@@ -248,7 +248,7 @@ class TestKnowledgeBaseSearchMiddlewarePlanner:
             return []
 
         async def fake_build_scoped_filesystem(**kwargs):
-            return {}
+            return {}, {}
 
         monkeypatch.setattr(
             "app.agents.new_chat.middleware.knowledge_search.search_knowledge_base",
@@ -298,7 +298,7 @@ class TestKnowledgeBaseSearchMiddlewarePlanner:
             return []
 
         async def fake_build_scoped_filesystem(**kwargs):
-            return {}
+            return {}, {}
 
         monkeypatch.setattr(
             "app.agents.new_chat.middleware.knowledge_search.search_knowledge_base",
@@ -334,7 +334,7 @@ class TestKnowledgeBaseSearchMiddlewarePlanner:
             return []
 
         async def fake_build_scoped_filesystem(**kwargs):
-            return {}
+            return {}, {}
 
         monkeypatch.setattr(
             "app.agents.new_chat.middleware.knowledge_search.search_knowledge_base",