fix(rstar): preserve signed and decimal answers during search

[liuzhengyang699]

R* extracts answers with \d+, so an explicit The final answer is -1.25 falls back to 25 at reduced confidence, and 1.75 becomes 1 at full confidence. This changes both rollout rewards and the answer-frequency vote, even when the model generated the correct value.

Use the same signed numeric pattern for explicit answers and the last-number fallback. Preserve decimal and scientific notation while retaining the existing confidence levels and integer behavior.

Validation: python -m pytest tests/test_rstar_numeric_answers.py tests/test_rstar_backpropagation.py tests/test_conversation_logging_approaches.py -q passed 15 tests and 34 subtests. The new cases cover extraction, rollout reward, decimal answer voting, and the full solve() path with mocked model responses. Before the fix, the negative-answer reward, decimal vote, and final output regressions fail. New-test Ruff and git diff --check passed.