🪨 fix: add AWS STS session token support to Bedrock client

[adamrb]

Summary

Add support for AWS Security Token Service (STS) temporary credentials in Bedrock client configuration. This enables the use of temporary security credentials through session tokens alongside standard AWS access keys.

Fixes #4391 & #4484

Change Type

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• Bug fix (non-breaking change which fixes an issue)

Testing

Tested with local environment variables set and the following config in docker-compose.override.yml before running 'docker compose up'

services:
  api:
    image: librechat
    build:
      context: .
      target: node
    environment:
      - BEDROCK_AWS_DEFAULT_REGION=us-west-2 # A default region must be provided
      - BEDROCK_AWS_ACCESS_KEY_ID=${AWS_ACCESS_KEY_ID}
      - BEDROCK_AWS_SECRET_ACCESS_KEY=${AWS_SECRET_ACCESS_KEY}
      - BEDROCK_AWS_SESSION_TOKEN=${AWS_SESSION_TOKEN}

Test Configuration:

Checklist

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• My code adheres to this project's style guidelines
• I have performed a self-review of my own code

[danny-avila]

Thanks!