feat: add AWS profile support for Bedrock credentials

[iElsha]

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Summary

This PR adds AWS profile support for Bedrock credentials, enabling automatic credential refresh using the AWS SDK's credential provider chain. This
improvement allows users to configure AWS profiles with optional credential_process for seamless token rotation, eliminating the need to manually
update static environment variables when credentials expire.

Key Changes:

• Added BEDROCK_AWS_PROFILE environment variable support
• Implemented AWS SDK credential provider chain following the existing S3 implementation pattern
• Updated credential loading logic to support three methods: user-provided credentials, static environment variables, and AWS profiles
• Added logging for credential source transparency
• Updated .env.example with comprehensive documentation on profile configuration and optional auto-refresh setup

Benefits:

• Automatic credential refresh for temporary credentials (no manual intervention needed)
• Follows AWS best practices for credential management
• Maintains backward compatibility with existing credential methods
• Supports IAM roles, EC2/ECS metadata, SSO, and AWS profiles

Change Type

Please delete any irrelevant options.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Testing

Please describe your test process and include instructions so that we can reproduce your test. If there are any important variables for your testing configuration, list them here.

Test Configuration:

Environment Variables:

BEDROCK_AWS_DEFAULT_REGION=us-west-2
BEDROCK_AWS_PROFILE=my-bedrock-profile

AWS Config (~/.aws/config):
[profile my-bedrock-profile]
region = us-west-2
credential_process = my-custom-refresh-command --format json

# This can also be   credential_process = node /path/to/LibreChat/config/get-aws-credentials.js or anything else...

Test Steps:

1. Set up AWS profile with credential_process in ~/.aws/config
2. Configure BEDROCK_AWS_PROFILE in .env file
3. Start LibreChat
4. Verify Bedrock endpoint connects successfully using profile credentials
5. Check logs confirm credential source: [Bedrock] Using AWS credential provider chain with profile: my-bedrock-profile
6. Test that credentials automatically refresh when expired

Backward Compatibility Testing:

• Verified existing static environment variable configuration still works
• Verified user-provided credentials flow unchanged
• Tested fallback to default credential provider chain when no profile specified

Checklist

Please delete any irrelevant options.

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• I have made pertinent documentation changes
• My changes do not introduce new warnings
• Local unit tests pass with my changes Test Suites: 1 skipped, 80 passed, 80 of 81 total
• A pull request for updating the documentation has been submitted.

[iElsha]

Issue: #10506