fix: fail fast on Tushare fundamental enrichment errors

[warren618]

Follow-up hardening for #76 by @mrbob-git. This keeps the new fundamental_fields Tushare statement enrichment feature credited to that PR while tightening the failure semantics around explicit statement-field requests.

Summary

• validate fundamental_fields as a table-to-field-list config shape
• fail fast when configured Tushare fundamental enrichment raises, instead of silently continuing with raw price bars
• add regressions for provider failure and malformed config

Validation

• python -m pytest agent/tests/test_engine_robustness.py agent/tests/test_tushare_fundamentals_provider.py agent/tests/test_fundamental_filter_example.py -q
• python -m ruff check agent/backtest/engines/base.py agent/backtest/runner.py agent/tests/test_engine_robustness.py agent/tests/test_tushare_fundamentals_provider.py agent/tests/test_fundamental_filter_example.py
• python -m py_compile agent/backtest/engines/base.py agent/backtest/runner.py agent/tests/test_engine_robustness.py agent/tests/test_tushare_fundamentals_provider.py agent/tests/test_fundamental_filter_example.py
• git diff --check