feat(agent): persist pending tool call in session snapshot

backend/src/models/agent_chat_session.py

@@ -14,7 +14,7 @@ from sqlalchemy import (
     func,
     text,
 )
-from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.dialects.postgresql import JSONB, UUID
 from sqlalchemy.orm import Mapped, mapped_column
 
 from core.db.base import Base, SoftDeleteMixin, TimestampMixin
@@ -75,3 +75,7 @@ class AgentChatSession(TimestampMixin, SoftDeleteMixin, Base):
     total_cost: Mapped[Decimal] = mapped_column(
         Numeric(12, 6), nullable=False, server_default=text("0")
     )
+    state_snapshot: Mapped[dict | None] = mapped_column(
+        JSONB,
+        nullable=True,
+    )