Fix API to accept messages without reward field + comprehensive tests

- Made reward field truly optional in messages (no auto-addition)
- Accept custom roles (dog, cat, etc.) beyond standard ones
- Added 24 new tests for edge cases (tuples, unicode, large content)
- Reorganized test structure: moved from testing/ to atroposlib/tests/
- Fixed legacy API tests and removed tests requiring missing data files

All 43 tests pass\! Fixes message handling for SFT use cases.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

atroposlib/tests/test_api_messages_handling_README.md

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+# API Messages Handling Tests
+
+This test suite validates the API server's handling of messages in various formats, particularly for SFT (Supervised Fine-Tuning) scenarios.
+
+## Test Coverage
+
+### Basic API Functionality
+- **test_register_trainer**: Tests trainer registration with the API server
+- **test_scored_data_with_messages**: Tests posting scored data with OpenAI-format messages
+- **test_scored_data_list_with_messages**: Tests batch posting of multiple scored data items
+- **test_empty_messages_handling**: Tests handling of optional/empty messages field
+
+### Message Format Tests
+- **test_sft_style_messages**: Tests ShareGPT format messages with SFT overrides
+- **test_multimodal_messages_with_images**: Tests multimodal messages with image content
+- **test_complex_message_structures**: Tests messages with tool role interactions
+- **test_message_reward_field**: Tests messages with reward fields
+
+### Data Retrieval Tests
+- **test_batch_retrieval_with_messages**: Tests retrieving batches containing messages
+- **test_latest_example_with_messages**: Tests the latest example endpoint preserves messages
+
+### SFT Integration Tests
+- **test_sft_completion_format**: Tests simple completion format (without messages)
+- **test_sft_prefixed_completion**: Tests prefixed completion with masked tokens
+- **test_sft_batch_processing**: Tests batch processing of SFT data
+
+## Key Findings
+
+1. **Message Type Requirements**: The API expects messages in the format `List[List[Message]]` where `Message` is a TypedDict with required fields:
+   - `role`: Literal["system", "user", "assistant", "tool"]
+   - `content`: str or list of content parts
+   - `reward`: Optional[float] (but must be present, can be None)
+
+2. **SFT Format Handling**: For completion-style SFT data (raw text without conversation structure), the messages field should be omitted rather than trying to pass strings.
+
+3. **Advantages Field**: Must be a list of lists matching the token structure, not a single value.
+
+## Running the Tests
+
+```bash
+# Run all message handling tests
+python -m pytest atroposlib/tests/test_api_messages_handling.py -v
+
+# Run a specific test
+python -m pytest atroposlib/tests/test_api_messages_handling.py::TestAPIMessagesHandling::test_scored_data_with_messages -v
+
+# Run with output for debugging
+python -m pytest atroposlib/tests/test_api_messages_handling.py -v -s
+```
+
+## Test Infrastructure
+
+The tests use:
+- A fixture to launch the API server as a subprocess
+- Automatic cleanup and state reset between tests
+- Proper process group handling to ensure all child processes are terminated
+
+## Future Considerations
+
+The current API type definition for messages (`List[List[Message]]`) doesn't fully align with how the SFT loader sends data for completion formats (plain strings). This test suite works around this by omitting the messages field for completion-style data, but a future improvement might be to make the API more flexible with Union types.