fix: preserve disabled session knowledge bases

[lxfight]

The session editor previously treated an empty knowledge base selection as missing configuration and deleted kb_config. The backend then inherited the global configuration, so a session that intentionally disabled all knowledge bases would search every globally enabled knowledge base.

Modifications / 改动点

• Add an explicit "Follow global knowledge base configuration" switch.
• Preserve an explicit empty kb_ids list when a session should use no knowledge bases.
• Delete the session override only when global inheritance is enabled.
• Disable knowledge base controls while the session inherits global settings.
• Add translations for English, Simplified Chinese, and Russian.
• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

Verification steps:

cd dashboard
pnpm build

Results:

• Vue TypeScript checks passed.
• Production Vite build passed (3,775 modules transformed).
• Manually verified the session rule editor can switch between global inheritance, selected knowledge bases, and an explicit empty selection without losing the override.

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。
• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。
• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。
• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Clarify session-level knowledge base behavior by adding explicit global inheritance controls and preserving empty knowledge base selections so sessions can intentionally disable all knowledge bases.

New Features:

• Introduce a switch to let sessions explicitly inherit the global knowledge base configuration.

Bug Fixes:

• Ensure sessions that select no knowledge bases preserve an explicit empty configuration instead of silently inheriting global settings.

Enhancements:

• Disable individual knowledge base controls when a session is configured to inherit global settings.
• Initialize and persist knowledge base configuration state based on whether a session overrides or inherits global rules.
• Add localized text for the new inheritance control in English, Simplified Chinese, and Russian.

[dosubot]

📄 Knowledge review

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[sourcery-ai]

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