Fix official backtests without feature pins

[pjy008008]

What changed

• allow official backtest requests with zero feature materialization pins
• add a regression test for strategies that require only market-bar inputs

Root cause

The Backend correctly emitted featureMaterializations: [] for strategies with no precomputed feature requirements. The Backtest consumer schema required at least one item, rejected the request as CONTRACT_VIOLATION, and moved it to the request DLQ while the run remained queued.

User impact

Basic strategies that do not require feature materializations can now pass request intake and proceed to execution.

Verification

• reproduced the failure before the fix: official_backtest_request.featureMaterializations: [] should be non-empty
• ruff check src tests
• full Docker-free pytest suite

Product authority instruction

Authority: user:pjy008008

이거 수정해줘 PR생성하고 CI통과하면 merge시켜줘.