Handle Dynamic Highlight Fields Transformation in azure-db

The current implementation of `transform_result` in our Azure Cognitive Search integration is hardcoded to search for "content" in `@search.highlights`. This approach is not flexible and fails to handle cases where `highlight_fields` is set to other fields or is `None`. We need to refactor this function to dynamically handle different `highlight_fields` and gracefully handle situations where `highlight_fields` is `None` (i.e., highlighting is not used).

## Tasks
- [ ] Modify `transform_result` to dynamically check for highlighted content based on the fields specified in `highlight_fields`.
- [ ] Implement a check in `transform_result` to bypass looking in `@search.highlights` when `highlight_fields` is `None`.
- [ ] Ensure that the original content of a field is used when its highlights are not available or when `highlight_fields` is `None`.
- [ ] Add support for handling multiple fields in `highlight_fields`.
- [ ] Write unit tests to cover various scenarios, including:
- `highlight_fields` set to different fields (other than "content").
- `highlight_fields` set to `None`.
- Multiple fields specified in `highlight_fields`.
- Absence of expected highlights in search results.

## Acceptance Criteria
- The `transform_result` function should dynamically handle the fields specified in `highlight_fields`, returning highlighted content appropriately.
- When `highlight_fields` is `None`, the function should not attempt to access `@search.highlights` and should use the original field content.
- The function must handle multiple highlight fields correctly, ensuring consistent results across different configurations.
- The solution must be robust, handling all specified edge cases, with unit tests confirming the expected behavior.