feat: upgrade LangGraph to 1.x and migrate deprecated APIs

- Bump langgraph from 0.4 to 1.0+, langgraph-supervisor from 0.0.12 to 0.0.30+
- Bump langchain-core, langchain-anthropic, langchain-openai to 1.x
- Add langchain>=1.0 dependency for new create_agent location
- Migrate create_react_agent -> create_agent (prompt -> system_prompt)
- Fix create_supervisor positional arg to named agents= parameter
- Replace AsyncMock checkpointer with InMemorySaver in tests (v1 type validation)
- Update version references in README, ARCHITECTURE, eng-review-plan

backend/tests/unit/test_graph.py

@@ -6,6 +6,7 @@ from typing import TYPE_CHECKING
 from unittest.mock import AsyncMock, MagicMock
 
 import pytest
+from langgraph.checkpoint.memory import InMemorySaver
 
 from app.graph import build_agent_nodes, build_graph, classify_intent
 from app.intent import ClassificationResult, IntentTarget
@@ -34,20 +35,20 @@ class TestBuildGraph:
         mock_llm = MagicMock()
         mock_llm.bind_tools = MagicMock(return_value=mock_llm)
         mock_llm.with_structured_output = MagicMock(return_value=mock_llm)
-        mock_checkpointer = AsyncMock()
+        checkpointer = InMemorySaver()
 
-        graph = build_graph(sample_registry, mock_llm, mock_checkpointer)
+        graph = build_graph(sample_registry, mock_llm, checkpointer)
         assert graph is not None
 
     def test_graph_has_classifier_attached(self, sample_registry: AgentRegistry) -> None:
         mock_llm = MagicMock()
         mock_llm.bind_tools = MagicMock(return_value=mock_llm)
         mock_llm.with_structured_output = MagicMock(return_value=mock_llm)
-        mock_checkpointer = AsyncMock()
+        checkpointer = InMemorySaver()
         mock_classifier = MagicMock()
 
         graph = build_graph(
-            sample_registry, mock_llm, mock_checkpointer, intent_classifier=mock_classifier
+            sample_registry, mock_llm, checkpointer, intent_classifier=mock_classifier
         )
         assert graph.intent_classifier is mock_classifier
         assert graph.agent_registry is sample_registry
@@ -56,9 +57,9 @@ class TestBuildGraph:
         mock_llm = MagicMock()
         mock_llm.bind_tools = MagicMock(return_value=mock_llm)
         mock_llm.with_structured_output = MagicMock(return_value=mock_llm)
-        mock_checkpointer = AsyncMock()
+        checkpointer = InMemorySaver()
 
-        graph = build_graph(sample_registry, mock_llm, mock_checkpointer)
+        graph = build_graph(sample_registry, mock_llm, checkpointer)
         assert graph.intent_classifier is None