fix: add safe dictionary access for bedrock credentials

[KMerdan]

Summary

This PR enhances error handling as discussed in Issue #11819 by implementing safe dictionary access for AWS credentials. Previously, the code relied on direct dictionary access, which risked KeyError exceptions if aws_region was missing in the provided credentials.

Changes made:

• Replace direct dictionary access with .get() method for aws_region
• Add validation check to ensure aws_region is provided
• Raise InvokeBadRequestError with a clear error message if the required credential is missing

This change makes the code more robust and provides better error handling for missing AWS region configuration.

Checklist

• This change requires a documentation update
• I understand that this PR may be closed in case there was no previous discussion or issues
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change
• I've updated the documentation accordingly
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[laipz8200]

LGTM