### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

When calling AWS Bedrock Mantle (the Anthropic-compatible endpoint, bedrock/mantle/anthropic.*), the aws_bedrock_project_id from litellm_params is sent to the Mantle Anthropic surface in an HTTP header named anthropic-workspace. AWS's Mantle endpoint expects the header anthropic-workspace-id (with the -id suffix). Because the header name is wrong, Mantle ignores it and the request runs at account-level data-retention settings instead of the project's.

The visible symptom is a confusing, unrelated-looking error for models that require an explicit data-retention mode (e.g. Claude Fable 5, whose allowed_modes is ["provider_data_share"] and which does not permit the implicit default mode):

BedrockException - {"type":"error","request_id":"req_...","error":{"type":"invalid_request_error","message":"data retention mode 'default' is not available for this model"}}

Even when the Bedrock project (proj_...) is correctly configured with provider_data_share, the call still fails, because the project selection header never reaches Mantle.

### S…