fix: avoid iterating invalid margin schedules

[stefan-jansen]

Summary

• treat only tuple/list values as margin schedule pairs during coercion, export, and validation
• serialize arbitrary invalid iterables as scalars without consuming them
• tighten regression coverage with a counting iterator that must not be read

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 tests/test_config_wiring.py::TestImmediateFill::test_validate_rejects_non_numeric_margin_pct_schedule -q
• pre-commit run --files src/ml4t/backtest/config.py tests/test_config_wiring.py

[Copilot]

Pull request overview

This PR updates BacktestConfig margin schedule coercion/serialization/validation to avoid accidentally iterating arbitrary iterables (e.g., generators/iterators) when handling invalid margin_pct_schedule values, preserving them as scalars during export and tightening regression coverage.

Changes:

• Treat only list/tuple values as (initial, maintenance) schedule pairs during coercion and validation.
• Serialize non-list/tuple invalid schedule values as scalars without consuming them.
• Add a regression test using a counting iterator to ensure export does not read invalid iterables.

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	Tightens margin schedule coercion/serialization/validation to avoid consuming arbitrary iterables.
tests/test_config_wiring.py	Adds regression coverage ensuring invalid iterable values are not consumed during to_dict() export.

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