fix(session_search): let unset mode flow to config-resolved default

The registry handler hardcoded mode=args.get("mode", "summary") and the
function signature defaulted to "summary", which together made the
tools.session_search.default_mode config knob structurally unreachable
from real tool calls — _resolve_user_default_mode() only fires when
mode is None/empty, but neither path ever delivered None.

Drop both "summary" fallbacks so an omitted mode flows through as None
and the config-resolution branch can run.

Adds two tests: a static guard on the registry handler source pattern
(mirroring the existing run_agent.py one) and an end-to-end regression
that dispatches through the registry with default_mode='fast' configured
and asserts result["mode"] == "fast".

tools/session_search_tool.py

@@ -745,7 +745,7 @@ def session_search(
     limit: int = 3,
     db=None,
     current_session_id: str = None,
-    mode: str = "summary",
+    mode: str = None,
     # Guided-mode-only parameters: anchored drill-down into one or more
     # session+message pairs. Required when mode='guided', ignored otherwise.
     # Use either the single-anchor pair (session_id + around_message_id) or
@@ -1209,7 +1209,7 @@ registry.register(
         query=args.get("query") or "",
         role_filter=args.get("role_filter"),
         limit=args.get("limit", 3),
-        mode=args.get("mode", "summary"),
+        mode=args.get("mode"),
         session_id=args.get("session_id"),
         around_message_id=args.get("around_message_id"),
         window=args.get("window", 5),