fix(history): ignore non-finite checkpoint timestamps

[Bluuok]

Summary

NaN or positive infinity in a checkpoint timestamp makes integer conversion raise. History projection then silently drops the message, while API history serialization raises instead of falling back to the row timestamp. Treat non-finite floats as missing timestamps so message content is retained and the existing fallback/live-stamping paths work. Integer inputs keep their existing behavior.

Target branch

• Base is develop
• Base is main

Type of change

• Bug fix
• Refactor / chore

Test plan

• uv run pytest tests/unit/history/test_nonfinite_checkpoint.py tests/unit/history/test_projection.py tests/unit/api/test_chat_polish.py -q: 50 passed. The corrected regression suite on unchanged base source gives 4 failed/3 passed: NaN and positive-infinity cases fail; negative infinity and the integer case already pass.
• make all: backend lint/typecheck and the non-live suite passed; 4047 passed, 137 skipped, 34 warnings.
• Enabled repository hook: make precommit and dashboard production build passed.
• Windows uses Python 3.12, PYTHONUTF8=1, and Git Bash. Live tests/external-provider calls were not run; Linux validation is left to PR CI.
• make all passes locally
• Added/updated tests

Checklist

• Updated CHANGELOG.md (if user-facing)
• README / docs updated (if needed)

[dangzitou]

Reviewed 8740abe559c286818055b98edff2d40cea209863 against upstream develop at 709f9986d26de8ba13134859e42aedced1d21977 in an isolated worktree (macOS, Python 3.12.14, dependencies installed with uv sync --locked --python 3.12).

The shared parser guard fixes both history projection and API timestamp fallback for NaN/+Infinity. Negative infinity was already rejected by the existing non-positive check. Restricting math.isfinite to floats also preserves the existing integer behavior, including integers too large for float conversion.

Independent validation:

• Ran the PR's seven regression cases against unchanged upstream source: 4 failed, 3 passed. The NaN/+Infinity projection and serialization cases fail before the fix.
• On this head, ran uv run pytest tests/unit/history/test_nonfinite_checkpoint.py tests/unit/history/test_projection.py tests/unit/api/test_chat_polish.py -q together with 21 additional reviewer cases: 71 passed. The extra cases cover normal seconds/milliseconds, invalid inputs, huge integers, human/assistant batch projection, deduplication, fallback timestamps, and preserving the original message when live-stamping a copy.
• make all PYTEST_JOBS=2 (using RUN="uv run --no-sync" after locked installation): passed, including lint/typecheck and 4172 passed, 18 skipped in the non-live suite. The checkout remained clean.

No blocking findings for this change. Real-provider calls were not exercised; GitHub returned no CI checks for this head at review time.