Fix stalled history compression when utility model fails

[fcsguyxcw]

Summary

• fall back to deterministic, bounded head-and-tail compression when the utility model raises or returns empty content
• keep the healthy utility-model path unchanged
• treat an empty bulk collection as a no-op instead of raising IndexError
• add regression coverage for exception, empty-response, compression-progress, and empty-bulk paths

Unlike catch-and-stop handling, the fallback produces a non-empty bounded summary so saturated histories can continue shrinking.

Tests

• .venv/Scripts/python.exe -m pytest tests/test_history_compression_fallback.py tests/test_history_compression_wait.py tests/test_chat_compaction.py -q — 9 passed
• .venv/Scripts/python.exe -m py_compile helpers/history.py tests/test_history_compression_fallback.py

Fixes #1881