fix(gcp): fail on missing or mixed SKU currency instead of seeding USD

[cristim]

What

Cloud SQL (providers/gcp/services/cloudsql) and Memorystore (providers/gcp/services/memorystore) pricing no longer seed the currency to USD and overwrite it with whichever SKU came last. Each SKU that contributes a price must carry a currency code, and all contributing SKUs must agree; otherwise the lookup returns an error. A consistent non-USD currency is returned as that currency. This follows the policy of the Azure fixes (#179-#187).

A small shared helper, providers/gcp/internal/billingcurrency.Unify, holds the rule and is used by both extractors.

Why

Same defect as the Azure half of #102: a missing or conflicting currency silently produced a mislabelled on-demand/commitment pair and a savings percentage computed across denominations.

Source of truth (Cloud Billing Catalog API, google.golang.org/api v0.274.0, cloudbilling/v1/cloudbilling-gen.go)

• Money.CurrencyCode ("three-letter ISO 4217 code") is per price: type Money at :763, field at :765.
• Prices live in PricingInfo (:980) -> PricingExpression (:914) -> TierRate (:1231) -> UnitPrice Money.
• The list call takes one request-level currencyCode (:3461-3463): "currency code for the pricing info in the response proto ... If not specified USD will be used." So one response is single-currency and conflicting or empty currencies among its SKUs are a data error, not a legitimate case.

Behaviour change

Callers of getSQLPricing / getRedisPricing (and so GetOfferingDetails) now get an error where they previously got a possibly wrong currency label. With no priced SKU, the extractor now returns an empty currency instead of "USD"; both callers already error on a zero on-demand price.

Scope

Refs #102, does not close it: the GCP Cloud Storage extractor (cloudstorage/client.go ~256/272) has the same shape but is also changed by open PR #205, so it is left for a follow-up. The Azure half is already on main. No request parameter was changed.

Verification

• New table tests TestGetSQLPricing_Currency and TestGetRedisPricing_Currency (fixtures only): consistent EUR returned as EUR, consistent USD, mixed USD/EUR, missing commitment currency, missing on-demand currency.
• On the pre-fix source the 3 mixed/missing cases fail in each package (2 consistent cases pass); after the fix all pass.
• go vet ./... and go test ./... in providers/gcp pass; golangci-lint (repo config, run from providers/gcp) exit 0, 0 issues.
• Not verified: no live Billing Catalog calls.

[cristim]

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📥 Commits

Reviewing files that changed from the base of the PR and between ea61b58 and a15d230.

📒 Files selected for processing (6)

• providers/gcp/internal/billingcurrency/billingcurrency.go
• providers/gcp/services/cloudsql/client.go
• providers/gcp/services/cloudsql/pricing_currency_test.go
• providers/gcp/services/memorystore/client.go
• providers/gcp/services/memorystore/client_coverage_test.go
• providers/gcp/services/memorystore/pricing_currency_test.go

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