fix: skip empty llm summaries

[he-yufeng]

Summary

• skip LLM context compression when the summary response is empty
• log a warning instead of inserting an empty summary placeholder into history
• add a regression test for blank completion_text

Closes #8133.

To verify

• .venv\Scripts\python.exe -m py_compile astrbot\core\agent\context\compressor.py tests\agent\test_context_manager.py
• .venv\Scripts\python.exe -m ruff check astrbot\core\agent\context\compressor.py tests\agent\test_context_manager.py
• .venv\Scripts\python.exe -m pytest tests\agent\test_context_manager.py -q --basetemp .tmp\pytest -p no:cacheprovider
• git diff --check

Summary by Sourcery

Handle empty LLM-generated summaries safely in the context compressor to avoid altering history when no usable summary is returned.

Bug Fixes:

• Prevent LLMSummaryCompressor from inserting an empty summary into the message history when the LLM returns blank or missing completion text.

Tests:

• Add an async regression test ensuring LLMSummaryCompressor preserves history and logs a warning when the LLM returns an empty summary.

[sourcery-ai]

Hey - I've left some high level feedback:

• Consider including contextual details (e.g., provider name or conversation identifier) in the warning log for empty summaries to make it easier to trace and debug in production.
• To avoid brittle coupling between the test and implementation, you could define the warning message as a constant and reference it both in LLMSummaryCompressor and in the test assertion instead of duplicating the literal string.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider including contextual details (e.g., provider name or conversation identifier) in the warning log for empty summaries to make it easier to trace and debug in production.
- To avoid brittle coupling between the test and implementation, you could define the warning message as a constant and reference it both in `LLMSummaryCompressor` and in the test assertion instead of duplicating the literal string.

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[gemini-code-assist]

Code Review

This pull request enhances the LLMSummaryCompressor to robustly handle empty or whitespace-only responses from the LLM. It now strips the completion text and returns the original message history if the summary is empty, accompanied by a warning log. Additionally, a new test case has been added to verify this fallback behavior. I have no feedback to provide as there were no review comments.