### Check for existing issues

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

### What happened?

On the **`bedrock/claude_platform/<model>`** route, when a `workspace_id` is supplied (which is **required** for this route), the value is correctly set as the `anthropic-workspace-id` HTTP header **but is ALSO serialized as a top-level `workspace_id` field inside the JSON body** POSTed to `…/v1/messages`.

`workspace_id` is not a valid field of the Anthropic Messages API request body. Anthropic's `/v1/messages` rejects unknown top-level fields, so the gateway (`aws-external-anthropic.<region>.api.aws`) is expected to return a `400 invalid_request_error`. The value ends up duplicated — once in the header (correct) and once in the body (incorrect).

This affects the **chat-completions surface only** (`litellm.completion(...)` and the proxy `/v1/chat/completions`, which uses `BedrockClaudePlatformConfig` → `AnthropicConfig`). The **native messages surface** (`litellm.anthropic_messages(...)` / `/v1/messages`, which uses `BedrockClaudePlatformMessagesConfig` → `AnthropicMessagesConfig`) is **not affected**, because it builds the body through an ex…