bug: Upgrade Langchain, AWS Powertools, Pydantic. Fix config without default embedding and step function deployment.

[charles-marion]

Issue #, if available: #596

Description of changes:

In #596 , the Step function prevents deployment because the account has reach the number of policies. See here
https://docs.aws.amazon.com/step-functions/latest/dg/sfn-best-practices.html#bp-cwl

The fix is to set the prefix of the log group names (/aws/vendedlogs/) so step function uses statement for all..

In #592, the CDK synth fails when there is no default embedding model (only needed when using Aurora/Opensearch).
Removed the check (since it's a valid use case) and verified the Embedding page is still working without default model.

Dependencies updates:
Because the build workflow fails due to a vulnerabilities in Langchain (https://github.com/aws-samples/aws-genai-llm-chatbot/security/dependabot/73), I upgraded the following

• Langchain: Guide
• Because it conflicts with Langchain, I also had to upgrade AWS Power tools
• Because both the above require a new version of pydantic, I also upgraded it. In this case, the project uses it mostly for validator which has a new bahevior for optional values. (Guide)

Testing
Manual testing + Integration test.

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[surukonda]

@charles-marion please make this change in lib/rag-engines/workspaces/delete-document.ts as well.

[charles-marion]

@charles-marion please make this change in lib/rag-engines/workspaces/delete-document.ts as well.

done