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

Apologies — this PR was auto-closed by GitHub when we force-pushed a history rewrite of this repository (repository maintenance; every commit SHA changed). That was not a judgment on this PR, and GitHub does not allow us to reopen it because the commits it was based on no longer exist in the new history.

If you'd like to continue with this change: rebase your branch onto the new main (or recreate it from a fresh clone) and open a new PR — feel free to link back to this one for context, and we'll pick up the review there.

Sorry for the churn, and thanks for contributing.