test: stabilize Phase 0 API and workflow tests [FASE-0.4]

Assert INVALID_TRANSITION JSON code on duplicate start, remove sys.modules stubs from T-106 tests, and complete boto3 stubs in integration tests.

backend/tests/test_integration_v2.py

@@ -115,7 +115,12 @@ for _mod in [
     "passlib", "passlib.context",
 ]:
     if _mod not in sys.modules:
-        sys.modules[_mod] = ModuleType(_mod)
+        m = ModuleType(_mod)
+        if _mod == "boto3":
+            m.client = MagicMock()
+        elif _mod == "botocore.exceptions":
+            m.ClientError = Exception
+        sys.modules[_mod] = m
 
 # Now safe to import
 from app.models.enums import TestState, TestResult, TechniqueStatus