fix: handle margin schedule float overflow

[stefan-jansen]

Summary

• catch OverflowError while coercing, serializing, and validating margin percentage schedules
• keep invalid overflow-triggering values YAML-safe for diagnostic export
• add regression coverage for overflow during float(...) conversion

Verification

• uv run pytest tests/test_config_wiring.py::TestImmediateFill::test_margin_pct_schedule_float_overflow_is_reported tests/test_config_wiring.py::TestImmediateFill::test_invalid_margin_pct_schedule_export_does_not_raise -q
• pre-commit run --files src/ml4t/backtest/config.py tests/test_config_wiring.py

[Copilot]

Pull request overview

This PR hardens BacktestConfig margin percentage schedule handling against OverflowError during float(...) coercion/validation/serialization, ensuring invalid values can still be exported safely for diagnostics.

Changes:

• Catch OverflowError while coercing and validating margin_pct_schedule values.
• Adjust margin schedule serialization to keep invalid values YAML-safe during to_dict() / to_yaml().
• Add regression tests covering overflow during float(...) conversion and YAML export safety.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/ml4t/backtest/config.py	Adds OverflowError handling for margin schedule coercion/validation and introduces helpers for YAML-safe serialization of invalid margin schedule values.
tests/test_config_wiring.py	Adds regression coverage ensuring overflow in __float__ is reported via validation and does not break YAML export.

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