feat(gcp/cuds): surface Compute Engine resource-based CUDs that cross-attribute to Cloud SQL / Memorystore CPU usage #4

Summary

Follow-up from LeanerCloud/cloud-commitments-cli#649 (closes LeanerCloud/cloud-commitments-cli#640). LeanerCloud/cloud-commitments-cli#649 made GCP Cloud SQL / Cloud Storage / Memorystore PurchaseCommitment advisory-only because their spend-based CUDs have no public create-API. That decision was correct for the spend-based product, but there is a separate, genuinely actionable optimization angle:

Google cross-attributes Compute Engine resource-based CUDs onto the underlying compute used by some managed services (Cloud SQL, Memorystore, Dataflow workers, GKE-on-CE nodes). The user CAN'T buy a "Cloud SQL CUD" via API, but they CAN buy a Compute Engine resource-based CUD (vCPU/RAM) for the region where their Cloud SQL or Memorystore instances run, and Google will automatically apply that discount to those workloads' underlying compute charges.

The Compute Engine resource-based CUD path has a programmatic API: compute.regionCommitments.insert (and globalCommitments for memory-only / GPU shapes).

What this issue is

Surface Compute Engine resource-based CUD recommendations sized to cover the CPU/RAM used by:

• Cloud SQL instances in the same region
• Memorystore for Redis / Valkey instances in the same region
• (optionally) Dataflow worker pools, GKE node pools running on CE

The PurchaseCommitment path for these recommendations would use the existing GCP Compute Engine CUD purchase code (which is already implemented for vanilla CE — verify by reading providers/gcp/services/compute/).

What this issue is NOT

• Not direct Cloud SQL CUD purchase (no API; fix(gcp/purchases): make CloudSQL/Storage/Memorystore commitments advisory-only (closes #640) cloud-commitments-cli#649 covers the advisory-only path for that)
• Not GCS storage discounts (no CUD product)
• Not the spend-based CUDs that the Console / gcloud surface — those stay advisory in fix(gcp/purchases): make CloudSQL/Storage/Memorystore commitments advisory-only (closes #640) cloud-commitments-cli#649

Suggested approach

1. Recommender wiring: GCP's Recommender API category google.cloud.recommender.ComputeEngineCommitmentRecommender already surfaces these resource-based CUD recommendations as a single category covering all the underlying compute. Check whether the existing CUDly GCP recommender integration includes that category or only the vanilla CE one.
2. Cross-attribution notation: when a CUD recommendation's projected coverage spans Cloud SQL / Memorystore / Dataflow workloads, surface that in the recommendation card so the user understands what they're actually buying. The frontend would show: "Buy Y vCPU + Z GB resource-based CUD in us-east1 — currently covers: 2 Cloud SQL instances + 1 Memorystore Redis + 4 GKE nodes" with the cross-attribution metadata.
3. Purchase path: reuse the existing CE CUD purchase code at providers/gcp/services/compute/client.go (or wherever the resource-based CUD purchase already exists). No new SDK needed.
4. Tests: assert that the resource-based-CUD recommendation surfaces with cross-attribution metadata for a mock account that has both CE and Cloud SQL workloads.

Acceptance criteria

• GCP recommendation collection includes Compute Engine resource-based CUDs sized to cover Cloud SQL / Memorystore / Dataflow underlying compute
• Recommendation surface (UI + email) explicitly notes the cross-attribution coverage so the user knows what's actually being discounted
• PurchaseCommitment uses the existing CE CUD API (no advisory-only path)
• Tests cover the cross-attribution metadata round-trip

Cross-references

• bug(gcp/purchases): CloudSQL/Storage/Memorystore PurchaseCommitment creates new billable resources instead of buying a commitment cloud-commitments-cli#640 (the bug that made fix(gcp/purchases): make CloudSQL/Storage/Memorystore commitments advisory-only (closes #640) cloud-commitments-cli#649 necessary)
• fix(gcp/purchases): make CloudSQL/Storage/Memorystore commitments advisory-only (closes #640) cloud-commitments-cli#649 (the advisory-only fix this builds on)
• feat(providers/azure): add RI utilization reporting (closes #558) cloud-commitments-cli#563 (Azure RI utilization — the parity counterpart)
• GCP docs: "Resource-based committed use discounts apply to CPU usage from Cloud SQL, Memorystore, Dataflow workers, and others" (verify exact list from current Google docs at implementation time — the cross-attribution list expands periodically)

Severity / Effort

• Severity: low — current advisory-only behavior is correct; this is upside, not a bug
• Effort: large — needs Recommender API extension, recommendation-card UX, and the cross-attribution data model