fix(ltm): Retain per-user contexts when enabling dialogue isolation

[Reisenbug]

开启群聊上下文感知的主动回复时，on_req_llm 会无条件执行 req.contexts = []， 导致对话隔离（unique_session）维护的 per-user 对话历史被清空，对话隔离无效

Modifications / 改动点

astrbot/builtin_stars/astrbot/long_term_memory.py： 仅在 unique_session 未开启时清空 req.contexts。开启时保留 per-user contexts，使其与群聊上下文背景并存。

• This is NOT a breaking change. / 这不是一个破坏性变更。

逻辑错误，无报错日志。可通过在 on_req_llm 执行前后打印 req.contexts 验证：unique_session 开启时执行后 contexts 不再被清空

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

Bug Fixes:

• Ensure unique_session conversation contexts are not cleared when active reply is enabled, so per-user dialog isolation remains effective.

[sourcery-ai]

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[gemini-code-assist]

Code Review

This pull request introduces a unique_session configuration to conditionally prevent clearing request contexts when active reply is enabled. However, the current implementation incorrectly attempts to retrieve this setting from a processed configuration object that lacks the required key, making the feature non-functional. A suggestion was provided to correctly access the configuration via the context.