feat: Implement backtest source management and enhance candle data handling
Changes:
- Introduced a new function `_apply_backtest_source_env_from_request` to manage the environment variables for candle, tick, and order book sources based on incoming requests.
- Added a teardown function `_teardown_backtest_source_env` to ensure that environment variables do not persist between requests, enhancing the stability of the backtesting environment.
- Refactored existing code to utilize the new source management functions, improving code readability and maintainability.
- Added new utility functions in `bt_candle_source.py` for fetching and managing candle data, ensuring consistency with live trading data sources.
Impact:
- These changes improve the flexibility and reliability of the backtesting framework, allowing for better management of data sources and reducing the risk of cross-request contamination.
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