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/multi_agent_chat/shared/date_filters.py

@@ -0,0 +1,63 @@
+"""
+Utility functions for SurfSense agents.
+
+This module provides shared utility functions used across the new_chat agent modules.
+"""
+
+from datetime import UTC, datetime, timedelta
+
+
+def parse_date_or_datetime(value: str) -> datetime:
+    """
+    Parse either an ISO date (YYYY-MM-DD) or ISO datetime into an aware UTC datetime.
+
+    - If `value` is a date, interpret it as start-of-day in UTC.
+    - If `value` is a datetime without timezone, assume UTC.
+
+    Args:
+        value: ISO date or datetime string
+
+    Returns:
+        Aware datetime object in UTC
+
+    Raises:
+        ValueError: If the date string is empty or invalid
+    """
+    raw = (value or "").strip()
+    if not raw:
+        raise ValueError("Empty date string")
+
+    # Date-only
+    if "T" not in raw:
+        d = datetime.fromisoformat(raw).date()
+        return datetime(d.year, d.month, d.day, tzinfo=UTC)
+
+    # Datetime (may be naive)
+    dt = datetime.fromisoformat(raw)
+    if dt.tzinfo is None:
+        return dt.replace(tzinfo=UTC)
+    return dt.astimezone(UTC)
+
+
+def resolve_date_range(
+    start_date: datetime | None,
+    end_date: datetime | None,
+) -> tuple[datetime, datetime]:
+    """
+    Resolve a date range, defaulting to the last 2 years if not provided.
+    Ensures start_date <= end_date.
+
+    Args:
+        start_date: Optional start datetime (UTC)
+        end_date: Optional end datetime (UTC)
+
+    Returns:
+        Tuple of (resolved_start_date, resolved_end_date) in UTC
+    """
+    resolved_end = end_date or datetime.now(UTC)
+    resolved_start = start_date or (resolved_end - timedelta(days=730))
+
+    if resolved_start > resolved_end:
+        resolved_start, resolved_end = resolved_end, resolved_start
+
+    return resolved_start, resolved_end