test: fix test compilation on Swift 6.3.3

[steipete]

Summary

• Replace unsupported swift-testing accuracy arguments in FireworksUsageFetcherTests with equivalent absolute-difference expectations.
• Cast caught any Error values to FireworksUsageError before asserting the exact expected cases.
• Update the scale-proof aggregate fixture for the current authoritative-cost and standard/priority token-split fields.

The Fireworks tests landed in #2687 after its CI runs were cancelled, so Swift 6.3.3 never compiled these assertions. Separately, the cost-split signature changed after the #2763 scale-proof tests landed, leaving that fixture call site stale.

This is test-only and unblocks CI test compilation for all in-flight PRs.

Validation

• swift build --build-tests
• swift test --filter "FireworksUsageFetcherTests|CostUsageStoreScaleProofTests" (12 tests passed)
• SwiftLint strict: 0 violations

make check reaches a pre-existing origin/main documentation consistency failure: provider manifests report 68 providers while README.md advertises 67. That unrelated file is intentionally outside this test-only patch.