docs: add pre/post processing docs for langchain python (#2378)

## Description

Trigger has been tested corresponding to local changes. Latest
successful run:
https://pantheon.corp.google.com/cloud-build/builds;region=global/1c37031f-95f1-4c6c-9ef8-0452277599d5?e=13802955&mods=-autopush_coliseum&project=toolbox-testing-438616

Note: After merging, update python pre and post processing sample
testing trigger.

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🛠️ Fixes #<issue_number_goes_here>

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Yuan Teoh <45984206+Yuan325@users.noreply.github.com>
Co-authored-by: Averi Kitsch <akitsch@google.com>

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

@@ -0,0 +1,51 @@
+# 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 asyncio
+import importlib
+import os
+from pathlib import Path
+
+import pytest
+
+ORCH_NAME = os.environ.get("ORCH_NAME")
+module_path = f"python.{ORCH_NAME}.agent"
+agent = importlib.import_module(module_path)
+
+GOLDEN_KEYWORDS = [
+    "AI:",
+    "Loyalty Points",
+    "POLICY CHECK: Intercepting 'update-hotel'",
+]
+
+# --- 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):
+        """Test that expected keywords are present in the script's output."""
+        output = script_output.out
+        print(f"\nAgent Output:\n{output}\n")
+        missing_keywords = [kw for kw in GOLDEN_KEYWORDS if kw not in output]
+        assert not missing_keywords, f"Missing keywords in output: {missing_keywords}"