fix(sdk): take max of OpenRouter cost and upstream_inference_cost

[nordicnode]

Summary

• user-facing credits were computed as usage.cost + cost_details.upstream_inference_cost at all three cost sites (sdk/src/impl/llm.ts streaming reportCost, generateText, and structured paths)
• usage.cost is the total amount charged and already includes the upstream portion reported separately in cost_details, per OpenRouter's usage accounting docs — summing them roughly doubles credits on normal (non-BYOK) OpenRouter routes, which lines up with the "extreme inflation" reports on Discord
• take the max instead, matching the server-side ledger's documented semantics (the Solar Pro 4 BYOK note in common/src/constants/freebuff-models.ts:313-320: cost = 0 there and upstream carries the real spend, so max is correct for both shapes)
• add a regression test asserting exact credits for cost=0.01, upstream=0.02 (fails under the sum: 3 vs 2)

Fixes #1164

Validation

• bun test sdk/src/impl/__tests__/usage-receipts.test.ts (5 pass, 1 new — verified the new test fails with the sum by stashing the fix)
• bun test sdk (489 pass; the 100 failures are network/env-dependent and pre-exist on main)
• bunx tsc --noEmit -p sdk/tsconfig.json clean