fix(gcp/computeengine): recommendations query a nonexistent recommender ID, so GCP CUD recs are always empty

[cristim]

Problem

providers/gcp/services/computeengine/client.go:394 lists recommendations from projects/<p>/locations/<region>/recommenders/google.billing.CostInsight.commitmentRecommender. That recommender ID doesn't exist in Google's recommender catalog (https://cloud.google.com/recommender/docs/recommenders). Resource-based CUD recommendations come from google.compute.commitment.UsageCommitmentRecommender; spend-based ones come from google.cloudbilling.commitment.SpendBasedCommitmentRecommender, which is parented on the billing account, not the project. So every region's ListRecommendations call fails whatever IAM is granted, the error is only warn-logged, and GCP Compute Engine CUD recommendations are silently empty.

Found while reviewing cloud-commitments-platform#377, whose roles/recommender.viewer grant covers the correct recommender (recommender.usageCommitmentRecommendations.list).

Fix

Use google.compute.commitment.UsageCommitmentRecommender for resource-based CUDs. If spend-based recommendations are wanted, query the billing-account-parented recommender separately. Add a test asserting the exact recommender path. Consider making a per-region failure that hits every region surface as an error instead of an empty result (fail loud).

[cristim]

Merged in #162 as 2d2116e23b7a4894da1e85cad2d32ee8e8448e4e.

Post-merge verification on macOS:

• Checked out the actual merge commit and confirmed its complete Git tree matches independently reviewed 6104f675db263c818760e3347cf2bb9ab3207a04 (tree fa725e63c22b79d4fd26aa465ddf9915e71b68c4).
• Ran GOWORK=off GOTOOLCHAIN=go1.26.6 go test -race -count=1 -run TestComputeRecommendationsThroughSDK . from providers/gcp: PASS, 2.280s.
• The public provider adapter exercised real Google SDK clients against local HTTP/gRPC fixtures, checking project/region recommender paths, routing metadata, pagination, state filtering, savings, empty results, all-region failures, and partial success. This is local SDK verification, not live Google backend validation. No cloud credentials or mutations were used.

The library fix is verified and this issue can remain closed. Application adoption is tracked separately in cloud-commitments-cli#2121, cloud-commitments-mcp#35, and cloud-commitments-platform#452; this result does not claim those consumers have adopted the fix.