refactor(agents): colocate KB-search tool with its sole consumer; fix report ImportError

shared/tools/knowledge_base.py had exactly one production consumer: the
report deliverable, which imported it via `from .knowledge_base import ...`
-- a sibling path that did not exist, so the report KB-search path would
raise ImportError at runtime.

Move the module next to report.py (subagents/builtins/deliverables/tools/)
which makes that relative import valid, and move its only dependency
(shared/utils.py date helpers) to multi_agent_chat/shared/date_filters.py,
shared between the KB tool and the knowledge_search middleware.

Drop the now-unused knowledge-base re-exports from the shared/tools barrel
and repoint the integration tests. import-all + error-contract stay green.

surfsense_backend/tests/integration/google_unification/conftest.py

@@ -239,7 +239,7 @@ def patched_shielded_session(async_engine, monkeypatch):
             yield session
 
     monkeypatch.setattr(
-        "app.agents.shared.tools.knowledge_base.shielded_async_session",
+        "app.agents.multi_agent_chat.subagents.builtins.deliverables.tools.knowledge_base.shielded_async_session",
         _test_shielded,
     )
 

surfsense_backend/tests/integration/google_unification/test_browse_includes_legacy_docs.py

@@ -17,7 +17,9 @@ async def test_browse_recent_documents_with_list_type_returns_both(
     committed_google_data, patched_shielded_session
 ):
     """_browse_recent_documents returns docs of all types when given a list."""
-    from app.agents.shared.tools.knowledge_base import _browse_recent_documents
+    from app.agents.multi_agent_chat.subagents.builtins.deliverables.tools.knowledge_base import (
+        _browse_recent_documents,
+    )
 
     space_id = committed_google_data["search_space_id"]