feat(api): Add chat_dashboard support to V2 API

[WangzJi]

• Add CHAT_DASHBOARD to ChatMode enum
• Enable chat_dashboard mode in V2 chat completions API
• Fix database config lookup to handle missing configurations gracefully

Description

This PR implements support for chat_dashboard mode in the v2 API, ensuring that the stream parameter works correctly in this mode. The implementation follows the same functionality as in v1 API but adapts it to the v2 API architecture and design patterns.

This PR addresses the issue where the dashboard mode was only available in v1 API, while project maintainers recommend using v2 API for new features.

Fixes #2697 #2699

How Has This Been Tested?

1. Setting up a local instance of DB-GPT
2. Making API calls to the v2 endpoint with chat_dashboard mode enabled
3. Verifying that the stream parameter works correctly in dashboard mode

Snapshots:

Checklist:

• My code follows the style guidelines of this project
• I have already rebased the commits and make the commit message conform to the project standard.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• Any dependent changes have been merged and published in downstream modules

[fangyinc]

LGTM.

[chenliang15405]

LGTM