fix: Adding detailed information on each metric evaluation

Additionally, few other small changes.
*   Updated a test fixture to support the latest eval data schema. Somehow I missed doing that previously.
*   Updated the `evaluation_generator.py` to use `run_async`, instead of `run`.
*   Also, raise an informed error when dependencies required eval are not installed.
*   Also, changed the behavior of AgentEvaluator.evaluate method to run all the evals, instead of failing at the first eval metric failure.

PiperOrigin-RevId: 775919127

src/google/adk/evaluation/constants.py

@@ -0,0 +1,20 @@
+# 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
+
+MISSING_EVAL_DEPENDENCIES_MESSAGE = (
+    "Eval module is not installed, please install via `pip install"
+    " google-adk[eval]`."
+)