fix(#7907): send_message_to_user cron 场景下 session 容错

[NayukiChiba]

Modifications / 改动点

Fixes #7907 : cron 等主动场景下，LLM 调用 send_message_to_user 时可能只传 session_id（如 oc_xxx）而非完整三段式 platform_id:message_type:session_id，导致 from_str() 解析失败，消息发送中断。

• astrbot/core/tools/message_tools.py：from_str() 失败时用 current_session
  补全 platform_id 和 message_type；优化工具描述减少 LLM 自行填写 session 的可能
• tests/unit/test_message_tools.py：新增 6 个测试用例覆盖完整/部分/默认 session 场景

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

tests/unit/test_message_tools.py::test_send_message_with_full_three_part_session PASSED
tests/unit/test_message_tools.py::test_send_message_with_partial_session_id_fallback PASSED
tests/unit/test_message_tools.py::test_send_message_defaults_to_current_session PASSED
tests/unit/test_message_tools.py::test_send_message_partial_session_falls_back_to_current PASSED
tests/unit/test_message_tools.py::test_cron_context_current_session_is_target_session PASSED
tests/unit/test_message_tools.py::test_send_message_empty_messages_returns_error PASSED

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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

Improve send_message_to_user session handling to be more robust when LLMs provide incomplete session identifiers, especially in cron-triggered contexts.

Bug Fixes:

• Allow send_message_to_user to reconstruct a full MessageSession from a bare session_id using the current session when parsing the provided session string fails.
• Return clearer session-related error messages when both direct parsing and fallback reconstruction fail.
• Ensure empty or missing messages are validated and reported as errors before attempting session resolution.

Enhancements:

• Clarify the send_message_to_user tool parameter description to discourage LLMs from manually specifying sessions unless a full three-part identifier is known.

Tests:

• Add unit tests covering full three-part sessions, partial session_id fallback using the current session, defaulting to the current session when none is provided, cron-specific session behavior, and error handling for empty messages.

[sourcery-ai]

Hey - I've left some high level feedback:

• When falling back to current_session in the except block, you now return error: invalid session: {session} for all failures; consider preserving or logging the original parsing error (or at least distinguishing between malformed three-part sessions and failed fallback reconstruction) to make debugging easier.
• The except Exception blocks around MessageSession.from_str and fallback reconstruction are quite broad; if possible, catch a narrower exception type (e.g., a specific MessageSession parsing error) to avoid masking unrelated errors.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- When falling back to `current_session` in the `except` block, you now return `error: invalid session: {session}` for all failures; consider preserving or logging the original parsing error (or at least distinguishing between malformed three-part sessions and failed fallback reconstruction) to make debugging easier.
- The `except Exception` blocks around `MessageSession.from_str` and fallback reconstruction are quite broad; if possible, catch a narrower exception type (e.g., a specific `MessageSession` parsing error) to avoid masking unrelated errors.

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[gemini-code-assist]

Code Review

This pull request updates the SendMessageToUserTool to handle partial session IDs by reconstructing the full session string using the current session's platform and message type. It also adds a comprehensive set of unit tests for these scenarios. Feedback was provided to improve the robustness of the session parsing logic, specifically by ensuring the fallback only occurs when the input lacks a colon and by handling cases where the session context might already be an object.