feat: capture provider cost details with three-tier dispatch

[harrisony]

Summary

• fix: add upstream_inference_cost fallback for total cost so BYOK requests (where usage.cost=0) report the real spend
• refactor: stop aliasing upstream_inference_prompt_cost → input_cost; keep upstream fields separate since upstream_inference_prompt_cost = input_cost + cached_input_cost (aliasing silently zeroed out costCached)
• feat: add three-tier cost dispatch in applyUsageCostDetails — superset (per-bucket gateway fields) → normal (upstream prompt/completions split with cache ratio) → minimal (proportional fallback)
• test: expand coverage with edge cases — zero-cost non-BYOK, OpenRouter markup (cost >> upstream sum), zero prompt tokens, heavy-cache-hit ratio split, end-to-end BYOK and non-BYOK flows

Test plan

• All 1378 backend tests pass (bun run test:force-all)
• Biome lint clean (bunx biome lint .)
• Each commit independently passes lint + tests (verified via git rebase --exec)

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