- Proxy decoded `model` from the encoded batch_id but never passed it
to `litellm.aretrieve_batch`.
- Without `model` in kwargs, litellm cannot load `BedrockBatchesConfig`
and falls into the legacy provider switch, which 400s for bedrock.
- Fix: set `data["model"] = model_from_id` before the litellm call in
the SCENARIO 1 (encoded batch_id) branch.
- Also corrects the error string in
`_handle_retrieve_batch_providers_without_provider_config` (said
`'create_batch'` despite being raised from the retrieve path).
- Adds tests covering retrieve + file_content round-trip for bedrock-
encoded IDs.
