test: bound invalid margin schedule iterator guard

[stefan-jansen]

Summary

• make the invalid margin schedule iterator regression fixture finite
• preserve the zero-read assertion so future eager materialization fails without hanging tests

Verification

• uv run pytest tests/test_config_wiring.py::TestImmediateFill::test_invalid_margin_pct_schedule_export_keeps_non_sequences_scalar -q
• pre-commit run --files tests/test_config_wiring.py

[Copilot]

Pull request overview

Updates a regression-test fixture in tests/test_config_wiring.py to ensure it can’t hang the test suite if code under test accidentally starts eagerly iterating an “invalid” margin schedule value. This supports the backtesting engine’s config serialization safety by keeping the “must not iterate non-sequence scalars” assertion while bounding worst-case iteration.

Changes:

• Make the CountingIterator fixture finite by raising StopIteration after 5 reads.
• Preserve the existing assertion that BacktestConfig.to_dict() must not consume the iterator (read_count == 0).

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