fix(gcp): price tiered SKUs from the first paid tier

[IAmJSD]

Root cause

extractSQLPriceFromSKU (Cloud SQL), extractPriceFromSKU (Memorystore) and extractStoragePriceFromSKU (Cloud Storage) priced a SKU from TieredRates[0] only. A SKU whose first tier is a free allowance ([0, >0]) came back as price 0, and the price == 0 → continue guard in each extract*PricingFromSKUs loop then dropped it, so the region/tier got no price instead of the paid one.

Fix

• New skumatch.PaidRate(*cloudbilling.PricingExpression) *cloudbilling.TierRate returns the first tier with a non-zero unit price (skipping nil tiers and tiers with no UnitPrice), or nil when every tier is free.
• The three extractors call it in place of TieredRates[0]. Everything after that (nanos math, Cloud Storage unit conversion, currency, slot) is unchanged.
• I picked "price from the first non-zero tier" (the first option in the issue's acceptance criteria). The catalog lists tiers by ascending StartUsageAmount, and a workload large enough to need a commitment is past the free allowance, so the first paid tier is the rate it pays at the margin. This is a documented assumption. I did not check the live Billing Catalog for a real Cloud SQL or Memorystore SKU shaped like this, and I made no GCP API calls.
• SKUs where every tier is free are still skipped, as before. A negative price is still non-zero, so the existing "negative catalog price" tests behave as before.

Closes #242

Verification

All of this is unit/fixture evidence built from hand-written cloudbilling.Sku values. No real GCP catalog data was used.

Fail-before (new tests run on main's extractors, before the fix):

--- PASS: TestPaidRate
--- FAIL: TestExtractSQLPricing_FreeFirstTierUsesPaidTier
--- PASS: TestExtractSQLPricing_AllTiersFreeIsSkipped
--- FAIL: TestExtractPricing_FreeFirstTierUsesPaidTier            (memorystore)
--- PASS: TestExtractPricing_AllTiersFreeIsSkipped                (memorystore)
--- FAIL: TestExtractStoragePricing_FreeFirstTierUsesPaidTier
      Max difference between 2.7397260273972603e-05 and 0 ...      (price was 0)
--- PASS: TestExtractStoragePricing_AllTiersFreeIsSkipped

(TestPaidRate tests the new helper, and the AllTiersFree tests are guards that pass before and after the fix.)

Pass-after, run from providers/gcp:

AWS_CONFIG_FILE=/dev/null AWS_SHARED_CREDENTIALS_FILE=/dev/null go test ./...                  # all ok
GOWORK=off AWS_CONFIG_FILE=/dev/null AWS_SHARED_CREDENTIALS_FILE=/dev/null go test -race -short ./...   # all ok
go build ./... && make vet                                                                       # clean
golangci-lint v2.10.1 run --allow-parallel-runners ./...                                         # 0 issues
gocyclo -over 10 internal/skumatch/skumatch.go services/{cloudsql,memorystore,cloudstorage}/client.go  # silent

I set AWS_CONFIG_FILE/AWS_SHARED_CREDENTIALS_FILE to /dev/null only because the local machine's default AWS profile is invalid. The GCP tests don't depend on it.

Mutation check: if PaidRate goes back to returning TieredRates[0], the three FreeFirstTierUsesPaidTier tests and the free first tier then paid cases in TestPaidRate fail.

Interaction with #424 (fix/issue-243)

#424 also edits providers/gcp/services/cloudstorage/client.go, in extractStoragePricingFromSKUs currency handling and the import block. This PR changes only the TieredRates[0] lines in extractStoragePriceFromSKU, adds no import there (skumatch is already imported), and puts its test in a new tiered_rates_test.go instead of #424's pricing_currency_test.go. A trial merge of fork/fix/issue-243 into this branch auto-merged with no conflicts, and go build ./... plus the cloudstorage/skumatch tests passed on the merged tree.

Out of scope

• providers/gcp/services/computeengine/pricing.go:259 also reads only TieredRates[0]. The issue doesn't list it, so I left it alone. It could switch to skumatch.PaidRate in a follow-up.
• Not checked against the real catalog: whether any Cloud SQL / Memorystore / Cloud Storage SKU actually has a [0, >0] tier shape.

Maintainer: please copy the issue's triage labels onto this PR: triaged, priority/p3, severity/low, urgency/eventually, impact/few, effort/s, type/bug.

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[cristim]

Independent adversarial review: CLEAN at 48dd6971 (claude-go-pr-merge-2, non-author reviewer). No blocking findings.

• PaidRate (skumatch.go:101-117) treats a tier as paid when Units or Nanos is non-zero (nanos-only prices count); all-free, nil expression, nil rate and nil UnitPrice return nil, and all three callers keep the previous "no price" handling. Price math Units + Nanos/1e9 and the tier's currency code are correct for Money (e.g. cloudsql/client.go:467-468).
• Fail-before: with origin/main's client.go for cloudsql, cloudstorage and memorystore, TestExtractSQLPricing_FreeFirstTierUsesPaidTier, TestExtractStoragePricing_FreeFirstTierUsesPaidTier and TestExtractPricing_FreeFirstTierUsesPaidTier fail; they pass at this head.
• Mutations: dropping the Nanos check fails TestPaidRate; iterating TieredRates[:0] fails TestPaidRate and cloudsql tests.
• Non-blocking: (1) PaidRate assumes tiers arrive sorted by StartUsageAmount; the API type (cloudbilling-gen.go:1232) does not promise order. Sorting before the scan would be cheap hardening; no known SKU violates it. (2) computeengine/pricing.go:253-265 still reads TieredRates[0] but fails loud on a zero price, and fix(gcp): pricing extractors read only TieredRates[0], so a SKU whose first tier is free is skipped #242 scopes only the three extractors changed here.
• Local verification (macOS, unit/mock, no GCP calls): (cd providers/gcp && go test -race -short ./internal/skumatch/... ./services/...) pass; make build, make vet pass.

Still needed before merge: owner approval of the fork CI runs and green CI on this SHA, and the author marking the PR ready. Labels mirrored from #242. CR skipped: quota/draft; the merge comment will carry CR waived: quota, adversarial review + local verification + green CI once CI is green.