feat: Adds Static User Simulator and User Simulator Provider

Details:
- Adds the `StaticUserSimulator` which implements the current functionality of supplying a fixed set of user prompts for an EvalCase.
- Adds the `UserSimulatorProvider` which determines the type of user simulator required for an EvalCase (StaticUserSimulator or LlmBackedUserSimulator).
- Integrates the UserSimulatorProvider and UserSimulator into the CLI and evaluation infrastructure.
- Updates and adds unit tests for the new functionality.
- Miscellaneous updates to lay groundwork for a full implementation of the LlmBackedUserSimulator in the future.
PiperOrigin-RevId: 822198401

tests/unittests/evaluation/test_user_simulator.py

@@ -0,0 +1,45 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import annotations
+
+from google.adk.evaluation.user_simulator import NextUserMessage
+from google.adk.evaluation.user_simulator import Status
+from google.genai.types import Content
+import pytest
+
+
+def test_next_user_message_validation():
+  """Tests post-init validation of NextUserMessage."""
+  with pytest.raises(
+      ValueError,
+      match=(
+          "A user_message should be provided if and only if the status is"
+          " SUCCESS"
+      ),
+  ):
+    NextUserMessage(status=Status.SUCCESS)
+
+  with pytest.raises(
+      ValueError,
+      match=(
+          "A user_message should be provided if and only if the status is"
+          " SUCCESS"
+      ),
+  ):
+    NextUserMessage(status=Status.TURN_LIMIT_REACHED, user_message=Content())
+
+  # these two should not cause exceptions
+  NextUserMessage(status=Status.SUCCESS, user_message=Content())
+  NextUserMessage(status=Status.TURN_LIMIT_REACHED)