fix(core): prefer provider-reported usage cost - #41933

Summary

• add an optional provider-reported USD cost to normalized AI usage
• pass OpenRouter streamed usage.cost through the OpenAI-compatible parser
• normalize GitHub Copilot total_nano_aiu to USD inside the Core AI SDK boundary
• prefer valid reported costs in session accounting while retaining catalog pricing as fallback
• carry reported cost through normal steps, title generation, and compaction

Details

• packages/ai only knows the normalized optional USD cost; Copilot billing units remain isolated to Core
• valid zero cost is authoritative
• missing, negative, NaN, or infinite cost falls back to token-derived catalog pricing

Testing

• bun typecheck from packages/ai
• bun typecheck from packages/core
• 51 focused Core tests across AI SDK conversion, usage accounting, runner settlement, title, and compaction
• focused OpenRouter streamed-cost test
• full monorepo pre-push typecheck: 32/32 tasks