feat: antropic model added fix & kb tooling fixes

- Updated main-agent middleware to clarify that both filesystem reads/writes and knowledge-base retrieval are handled by the `knowledge_base` subagent.
- Introduced `_forward_mention_pins` function to carry `@`-mention pins into subagent state.
- Revised system prompts to reflect the new retrieval method and ensure proper citation handling.
- Removed the `search_knowledge_base` tool and its related tests, consolidating functionality under the `task` tool.
- Enhanced documentation to guide usage of the new retrieval approach and citation practices.

surfsense_backend/tests/unit/agents/chat/runtime/referenced_chat_context/test_transcript.py

@@ -7,8 +7,8 @@ import pytest
 from app.agents.chat.runtime.referenced_chat_context import (
     ReferencedChat,
     render_referenced_chats_block,
+    transcript as transcript_mod,
 )
-from app.agents.chat.runtime.referenced_chat_context import transcript as transcript_mod
 from app.agents.chat.runtime.referenced_chat_context.models import ReferencedChatTurn
 
 pytestmark = pytest.mark.unit
@@ -77,9 +77,7 @@ def test_oversized_single_turn_is_partially_filled_to_use_budget(
 ) -> None:
     monkeypatch.setattr(transcript_mod, "_MAX_CHARS_PER_REFERENCE", 40)
 
-    block = render_referenced_chats_block(
-        [_chat(1, "T", [("assistant", "x" * 500)])]
-    )
+    block = render_referenced_chats_block([_chat(1, "T", [("assistant", "x" * 500)])])
 
     assert block is not None
     # The turn is too big to keep whole, so its tail fills the budget with a

surfsense_backend/tests/unit/agents/chat/runtime/test_llm_config_sanitizer.py

@@ -3,13 +3,28 @@
 from __future__ import annotations
 
 import pytest
-from langchain_core.messages import AIMessage
+from langchain_core.messages import AIMessage, SystemMessage
 
 from app.agents.chat.runtime.llm_config import _sanitize_messages
 
 pytestmark = pytest.mark.unit
 
 
+def test_sanitize_messages_drops_whitespace_only_system_text_block() -> None:
+    # Mirrors TodoListMiddleware appending ``{"type":"text","text":"\n\n"}`` to
+    # the system message: Anthropic rejects whitespace-only system blocks.
+    original = SystemMessage(
+        content=[
+            {"type": "text", "text": "real system prompt"},
+            {"type": "text", "text": "\n\n"},
+        ]
+    )
+
+    sanitized = _sanitize_messages([original])
+
+    assert sanitized[0].content == "real system prompt"
+
+
 def test_sanitize_messages_strips_provider_specific_thinking_blocks() -> None:
     original = AIMessage(
         content=[