### Check for existing issues

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

### What happened?

All `bedrock-mantle` code paths (`bedrock_mantle/` standalone provider and `bedrock/mantle/` route) sign SigV4 requests with `service_name="bedrock"`, but the `bedrock-mantle` endpoint is a separate AWS service that requires `service_name="bedrock-mantle"` in the SigV4 credential scope.

Evidence: - The AWS-managed IAM policy [`AmazonBedrockMantleInferenceAccess`](https://console.aws.amazon.com/iam/home#/policies/arn:aws:iam::aws:policy/AmazonBedrockMantleInferenceAccess) uses `bedrock-mantle:*` actions (not `bedrock:*`) - CloudTrail shows the event source as `bedrock-mantle.amazonaws.com` (not `bedrock.amazonaws.com`) - The [`AmazonBedrockMantleFullAccess`](https://console.aws.amazon.com/iam/home#/policies/arn:aws:iam::aws:policy/AmazonBedrockMantleFullAccess) managed policy uses `bedrock-mantle:*`

This means when LiteLLM signs with `service_name="bedrock"`, AWS IAM evaluates the request against the `bedrock` service namespace, finds no matching allow, and rejects with a permission error even when `bedrock-mantle:CreateInference` is granted. …