fix(gcp/computeengine): report monthly CUD cost as recurring, not upfront

[cristim]

What

Align Compute Engine CUD cost fields with the shared semantics (CommitmentCost = upfront, 0 for monthly or no-upfront; RecurringMonthlyCost = USD per month). Before, a monthly rec had CommitmentCost = the term total and RecurringMonthlyCost = total / months.

The platform currently computes upfront + monthly x term (platform scheduler.go:1650), so the old output was counted about 2x. This PR removes that overcount. platform#683: please confirm for the platform owner that no other consumer relies on CommitmentCost being the term total for GCP.

• enrichRecWithPricing sets RecurringMonthlyCost = committed term total / months and leaves CommitmentCost 0. A pricing failure leaves RecurringMonthlyCost nil (unknown), not 0.
• BreakEvenMonths is no longer computed here: with no upfront payment it is undefined, and the old value treated the whole term cost as sunk.
• OnDemandCost is unchanged (on-demand price over the term).
• GCP CUDs are billed monthly (https://cloud.google.com/docs/cuds), so the forced monthly payment option stays.

Stacked on go#381

This branch contains the go#381 commits (pricing); merge #381 first, then this one rebases cleanly onto main. Only the last commit is new.

Fail before, pass after

TestConverterFillsPricingForScorer (4 vCPU, 6 GiB, live-shaped N1 1-year SKUs) on the pre-change code:

Error:      Should be zero, but was 838.10424
Messages:   a monthly CUD has no upfront payment, so CommitmentCost is 0 (...)
Error:      Should be zero, but was 20.431520501138948
Messages:   no upfront payment, so there is nothing to break even on

After: passes, with RecurringMonthlyCost = (4 x 0.019915 + 6 x 0.002669) x 8760 / 12 = 69.84 USD/month. go test ./... in providers/gcp passes.

Closes #383
Refs platform#683

🤖 Generated with Claude Code

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[cristim]

Independent review (claude-opus-5-5) of 2112144: CHANGES REQUESTED

Reviewed head 2112144 (0fcebc2 plus a merge of go#381's 1a25519).

The CommitmentCost and RecurringMonthlyCost change is right. Docs (https://docs.cloud.google.com/docs/cuds, redirected from cloud.google.com/docs/cuds): "You are billed a monthly fee for the commitments you purchase." The page mentions no upfront option. So CommitmentCost = 0, RecurringMonthlyCost = committed hourly x 8760 x years / (12 x years) = hourly x 730, and the forced monthly all match the shared semantics. The platform computes upfront + monthly x term (platform internal/scheduler/scheduler.go, UpfrontCost: rec.CommitmentCost / MonthlyCost: rec.RecurringMonthlyCost), so a 1yr n2-standard-8 now totals 0 + 178.65 x 12 = 2143.82, where it used to be 2 x 2143.82.

Blocking

1. client.go enrichRecWithPricing: rec.OnDemandCost = pricing.OnDemandPrice is the on-demand price over the whole term. AWS (parser_ri.go, EstimatedMonthlyOnDemandCost; parser_sp.go hourly x 730) and Azure (consumption.go, x DaysPerMonth / lookBack) report it monthly. The platform also reads it as "the canonical on-demand monthly baseline" (internal/config/types.go:592) and computes savings % = savings / on_demand_cost (frontend/src/approval-details.ts:296). Live, end to end, for n2-standard-8 in us-central1: OnDemandCost = 3403.01 (1yr) and 10209.04 (3yr), where the monthly figure is 0.388472 x 730 = 283.58. That makes the platform's savings % 12x or 36x too small. This PR is the cost-semantics change and states that OnDemandCost is left unchanged; it needs to be on-demand hourly x 730 as well.

Non-blocking
2. Coverage gap: changing monthly := pricing.CommitmentPrice / float64(termYears*12) to / 12 survives the suite, because TestConverterFillsPricingForScorer is 1yr only. A 3yr case would pin it. The live 3yr value is 0.174824 x 730 = 127.62.
3. Cloud SQL and Memorystore still use the old semantics (CommitmentCost = term total, BreakEvenMonths). Needs a follow-up issue.
4. GetOfferingDetails falls through to upfrontCost = totalCost for any payment option other than monthly or upfront. It is unreachable from the converter (which forces monthly), and it predates this PR.

Evidence

• Fail before: go#385's client_test.go on go#381's source gives Should be zero, but was 838.10424 and Should be zero, but was 20.431520501138948, the same as the PR body.
• Live end to end (real billing API, read-only; go#381 + fix(gcp/computeengine): report monthly CUD cost as recurring, not upfront #385 + fix(gcp/computeengine): normalize recommender savings by the projection duration #386 + fix(gcp/computeengine): follow every page of the billing SKU listing #388 merged locally): 1yr CommitmentCost=0 RecurringMonthly=178.651440 BreakEven=0; 3yr RecurringMonthly=127.621520. Both match a hand calculation from the live SKUs.
• Mutation monthly := pricing.CommitmentPrice (the term total) is killed.
• Builds, vet and tests pass. CI on 2112144: only CodeRabbit reports (pass), because the base is go#381's branch, so the full workflow set has not run on this head. No conflict with go#307.