fix(history): retain recent messages during summarization

[dexhunter]

Summary

• restore the newest budget-fitting head suffix during chat summarization
• preserve chronological order, the unsummarized tail, and existing recursion
• cover exact boundaries, nonpositive budgets, call count, and large histories

PR #3764 replaced the previous reverse/select/reverse logic with forward iteration. That also changed selection from the newest head suffix to the oldest prefix, so recent eligible messages could be omitted while much older messages were retained.

Fixes #5440.

Autoresearch

Running autoresearch with Weco reproduced the regression and tested candidate fixes against deterministic context-retention and scaling gates:

https://dashboard.weco.ai/share/DrDbVZz3X8a69GvOUew-jxkxDJw0aE7N

• context_retention_score: 20.0 to 100.0
• 14 repeated strict candidate evaluations: 100.0
• linear scaling retained through a 50,000-message fixture

Validation

• pytest tests/basic/test_history.py tests/basic/test_coder.py -q: 49 passed, 29 subtests passed
• focused regression tests: 8 passed, 4 subtests passed
• Ruff and flake8 pass
• git diff --check passes

[CLAassistant]

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