fix(gcp): match SKUs by whole tier token and explicit region

[cristim]

What

Cloud SQL, Memorystore and Compute Engine SKU matching now (1) requires the tier or machine type to appear as a whole token in the description and (2) applies a SKU to a region only when that region is listed in ServiceRegions, or when the list is empty and GeoTaxonomy.Type is GLOBAL. Both rules live in the new providers/gcp/internal/skumatch.

Why

A SKU with nil ServiceRegions priced every region, and the tier test was a plain substring (db-n1-standard-1 matched db-n1-standard-16, STANDARD matched STANDARD_HA).

The catalog docs say an empty region list means global: "This field is empty for Global SKUs since they are associated with all Google Cloud regions" (cloud.google.com/billing/v1/how-tos/catalog-api) and "Empty for Global skus, which are associated with all Google Cloud regions" (services.skus.list reference). The SDK's GeoTaxonomy.Type documents GLOBAL, REGIONAL and MULTI_REGIONAL (google.golang.org/api v0.274.0, cloudbilling/v1/cloudbilling-gen.go:611-624). So an empty list is trusted only with a GLOBAL taxonomy. Empty regions with REGIONAL, MULTI_REGIONAL, unspecified or a missing taxonomy is ambiguous and no longer matches.

Behaviour change

Callers get no match where a region-less SKU of unknown scope used to match every region.

Limits

Real catalog descriptions do not contain tiers or machine types (for example "Cloud SQL for MySQL: Regional - 1 vCPU + 3.75GB RAM in Americas", see cloud.google.com/skus/sku-groups/cloud-sql-cud-eligible-skus). Real lookups are therefore unchanged by this PR and still return no pricing for the token-matched services; the whole-token matching is correct on synthetic strings only. This PR does not claim a price correction. The real-data matching problem is tracked separately. Cloud Storage is untouched because #205 edits that file.

Verification

• Tests cover nil, empty and named regions across GLOBAL, REGIONAL, MULTI_REGIONAL, unspecified and missing taxonomy, plus decoy descriptions.
• Mutations of InRegion (inverted GLOBAL check; REGIONAL treated as global) each fail the skumatch tests.
• go vet ./..., go test ./..., golangci-lint run ./... in providers/gcp all exit 0.

Refs #97

[cristim]

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📥 Commits

Reviewing files that changed from the base of the PR and between be54dfa and cf4874b.

📒 Files selected for processing (9)

• providers/gcp/internal/skumatch/skumatch.go
• providers/gcp/internal/skumatch/skumatch_test.go
• providers/gcp/services/cloudsql/client.go
• providers/gcp/services/cloudsql/client_test.go
• providers/gcp/services/computeengine/client.go
• providers/gcp/services/computeengine/client_test.go
• providers/gcp/services/memorystore/client.go
• providers/gcp/services/memorystore/client_coverage_test.go
• providers/gcp/services/memorystore/client_test.go

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📝 Walkthrough

Walkthrough

The change adds shared helpers for matching SKU description tokens and service regions. Cloud SQL, Compute Engine, and Memorystore use these helpers. Tests cover token boundaries, region matching, and Cloud SQL pricing extraction.

Changes

GCP SKU Matching

Layer / File(s)	Summary
Shared SKU matching helpers providers/gcp/internal/skumatch/*	Adds case-insensitive service-region matching and token matching that rejects partial matches. Tests cover exact and case-insensitive matches, token boundaries, empty tokens, and absent regions.
Service SKU matching providers/gcp/services/cloudsql/*, providers/gcp/services/computeengine/*, providers/gcp/services/memorystore/*	The three services use the shared matching helpers. Tests cover regionless SKUs and tier-prefix decoys. Cloud SQL pricing extraction is tested with exact-tier, larger-tier, and regionless SKUs.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to cf487

This change makes GCP SKU matching stricter, so tiers and regions are matched exactly. No actionable merge-blocking risk was identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 35.71% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 14 functions across 9 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: matching SKUs by whole tier tokens and explicitly listed regions.

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