fix: keep invalid margin schedules scalar

[stefan-jansen]

Summary

• serialize invalid margin schedule values element-wise only for tuple/list values
• keep strings, mappings, and iterators scalar to avoid corrupting export output or consuming arbitrary iterables
• add regression coverage for string, dict, and generator invalid schedule values

Verification

• uv run pytest tests/test_config_wiring.py::TestImmediateFill::test_invalid_margin_pct_schedule_export_keeps_non_sequences_scalar 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 adjusts BacktestConfig.to_dict() export behavior for invalid margin_pct_schedule entries so that only list/tuple invalid values are serialized element-wise, while strings, mappings, and iterators are preserved as scalars to avoid corrupting exports or consuming arbitrary iterables.

Changes:

• Update invalid margin schedule serialization to only expand list/tuple values element-wise.
• Keep non-list/tuple invalid values (e.g., str, dict, generators) scalar via _serialize_invalid_margin_scalar.
• Add regression coverage for invalid schedule values that are non-sequences (string, dict, generator).

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	Changes invalid margin schedule serialization to only iterate list/tuple values.
tests/test_config_wiring.py	Adds a regression test for exporting invalid schedule values that are non-sequence types.

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