fix: preserve chat history when background summarization fails

[gary23w]

Summary

When background chat summarization raises ValueError, the worker reports a warning but leaves summarized_done_messages empty. On joining the worker, summarize_end() then replaces unchanged, intact chat history with that empty list.

Retain the original snapshot as the fallback on failure. The existing comparison still prevents replacement if history changed during summarization, and a later successful summary can still replace matching history. No changes to summarization prompts, model selection, or retry policy.

Verification

• The new regression exercises the real background-thread/start/join lifecycle with a stub summarizer: a failure preserves the original messages and warns; a subsequent success replaces them with the summary.
• The regression fails on the original source because history becomes [].
• python -m pytest tests/basic/test_coder.py tests/basic/test_history.py -q: 49 passed, 29 subtests passed on Windows. Temporary Git fixture commits used process-local commit.gpgsign=false; no user Git configuration was changed.
• Black, isort, flake8, and git diff --check pass.
• No live model completion calls.

AI assistance: Codex helped identify and reproduce the defect, implement the one-line fallback, independently review the diff, and run tests/checks.

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