fix(cost): prefer authoritative batch item usage

[seonghobae]

Summary

• prefer authoritative batch-item usage when trace or race bookkeeping exists but carries no provider usage
• keep per-step accounting unchanged when any billable step reports usage
• add a regression covering both trace and race_usage paths

Testing

• uv run pytest tests/test_cost_router.py -k 'batch_item_usage_wins_when_trace_and_race_steps_lack_usage or batch_completion_records_on_retrieve or invalid_batch_usage or provider_confirmed_zero_usage_stays_measured_and_price_known' -q
• uv run pytest tests/test_cost_router.py tests/test_cost_router_boundaries.py -q

Closes #1018

[coderabbitai]

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Reviewing files that changed from the base of the PR and between 2e414d1 and 9158a7c.

📒 Files selected for processing (2)

• contextual_orchestrator/cost_router.py
• tests/test_cost_router.py

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• Head SHA: 9158a7c2ceb5812faef3d3c659d8cf4f3c0ed7c1