fix:Error when configuring data sources via environment variables (env)

[liuhill]

Description

 When attempting to configure data sources for DB-GPT using environment variables (env), an error is thrown, preventing successful connection to the target data source and disrupting core functionality. This issue occurred due to improper parsing of env variables, missing validation for required configuration fields, or incorrect mapping between env entries and the application’s data source configuration logic.
  This PR addresses the root cause of the env-based data source configuration failure, ensuring users can reliably set up data sources via environment variables without errors.

How Has This Been Tested?

• configure .env file

MYSQL_PASSWORD=123456

• Then run the following command to start the webserver:
  uv run --env-file .env dbgpt start webserver --config dbgpt-proxy-siliconflow-mysql.toml

• Verified successful data source connection and query execution after applying env configurations.

Snapshots: