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/app/agents/shared/middleware/knowledge_search.py

@@ -41,6 +41,10 @@ from litellm import token_counter
 from pydantic import BaseModel, Field, ValidationError
 from sqlalchemy import select
 
+from app.agents.multi_agent_chat.shared.date_filters import (
+    parse_date_or_datetime,
+    resolve_date_range,
+)
 from app.agents.shared.feature_flags import get_flags
 from app.agents.shared.filesystem_selection import FilesystemMode
 from app.agents.shared.filesystem_state import SurfSenseFilesystemState
@@ -49,7 +53,6 @@ from app.agents.shared.path_resolver import (
     build_path_index,
     doc_to_virtual_path,
 )
-from app.agents.shared.utils import parse_date_or_datetime, resolve_date_range
 from app.db import (
     NATIVE_TO_LEGACY_DOCTYPE,
     Chunk,