docs: add pre/post processing docs for langchain python

docs/en/samples/pre_post_processing/python/agent_test.py

@@ -0,0 +1,58 @@
+# Copyright 2026 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.
+
+import os
+import pytest
+from pathlib import Path
+import asyncio
+import sys
+import importlib.util
+
+ORCH_NAME = os.environ.get("ORCH_NAME")
+module_path = f"python.{ORCH_NAME}.agent"
+agent = importlib.import_module(module_path)
+
+
+@pytest.fixture(scope="module")
+def golden_keywords():
+    """Loads expected keywords from the golden.txt file."""
+    golden_file_path = Path(__file__).resolve().parent.parent / "golden.txt"
+    if not golden_file_path.exists():
+        pytest.fail(f"Golden file not found: {golden_file_path}")
+    try:
+        with open(golden_file_path, 'r') as f:
+            return [line.strip() for line in f.readlines() if line.strip()]
+    except Exception as e:
+        pytest.fail(f"Could not read golden.txt: {e}")
+
+
+# --- Execution Tests ---
+class TestExecution:
+    """Test framework execution and output validation."""
+
+    @pytest.fixture(scope="function")
+    def script_output(self, capsys):
+        """Run the agent function and return its output."""
+        asyncio.run(agent.main())
+        return capsys.readouterr()
+
+    def test_script_runs_without_errors(self, script_output):
+        """Test that the script runs and produces no stderr."""
+        assert script_output.err == "", f"Script produced stderr: {script_output.err}"
+
+    def test_keywords_in_output(self, script_output, golden_keywords):
+        """Test that expected keywords are present in the script's output."""
+        output = script_output.out
+        missing_keywords = [kw for kw in golden_keywords if kw not in output]
+        assert not missing_keywords, f"Missing keywords in output: {missing_keywords}"