[Python GenAI Sample App] Temporarily hardcode langchain to older working version 0.3.27 (#472)

*Issue description:*
- python has genai workflow failures
https://github.com/aws-observability/aws-otel-python-instrumentation/actions/runs/18730013353/job/53426255842
- Looks like there are a lot of Langchain changes around the time of
when the failures started happening:
https://pypi.org/project/langchain/#history

*Description of changes:*
- Temporarily hardcode langchain to older working version 0.3.27

*Rollback procedure:*

We safely revert this commit if needed.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.

Author: jj22ee

terraform/python/ec2/adot-genai/main.tf

@@ -84,7 +84,9 @@ unzip genai-service.zip
 python3.12 -m pip install fastapi uvicorn[standard] --no-cache-dir
 python3.12 -m pip install boto3 botocore setuptools --no-cache-dir
 python3.12 -m pip install opentelemetry-api opentelemetry-sdk opentelemetry-semantic-conventions --no-cache-dir
-python3.12 -m pip install langchain langchain-community langchain_aws --no-cache-dir
+
+# Temporarily hardcode langchain to 0.3.27, as we suspect later versions break the canary tests for gen-ai
+python3.12 -m pip install langchain==0.3.27 langchain-community langchain_aws --no-cache-dir
 python3.12 -m pip install python-dotenv openlit --no-cache-dir
 python3.12 -m pip install openinference-instrumentation-langchain --no-cache-dir
 ${var.get_adot_wheel_command}