feat: add permission check for /new command

[Rain-0x01-39]

/new 命令缺少权限控制，在群聊且会话隔离关闭的场景下（共享会话），任意成员都可以创建新会话并切换，影响其他用户的对话上下文。而 /reset 已有对应的权限检查，/new 与之不一致。

Modifications / 改动点

• astrbot/builtin_stars/builtin_commands/commands/conversation.py：为 new_conv 方法新增场景化权限检查，逻辑与 reset 一致——群聊+会话隔离关闭时默认要求 admin，其余场景 member 即可。权限可通过 Dashboard 的指令管理覆盖。

• 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

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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

Enforce context-aware permissions for creating new conversations.

Bug Fixes:

• Restrict the /new command in shared group conversations to administrators, preventing members from changing the shared conversation context.

Enhancements:

• Align /new permission behavior with /reset while preserving configurable per-scenario permission overrides.

[sourcery-ai]

Hey - I've left some high level feedback:

• The new permission check for /new duplicates logic that likely already exists for /reset; consider extracting a shared helper or reusing the existing mechanism so the two commands stay consistent and easier to maintain.
• Role and permission values ("admin"/"member") are currently hard-coded string literals; it would be more robust to use centralized constants or an enum and validate against known roles to avoid subtle bugs from typos or future changes.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new permission check for `/new` duplicates logic that likely already exists for `/reset`; consider extracting a shared helper or reusing the existing mechanism so the two commands stay consistent and easier to maintain.
- Role and permission values ("admin"/"member") are currently hard-coded string literals; it would be more robust to use centralized constants or an enum and validate against known roles to avoid subtle bugs from typos or future changes.

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