fix(marketplace): fail loud instead of listing no-upfront RIs at $0

[cristim]

Summary

Follow-up to #808 (Sell-on-Marketplace feature), found during an adversarial review sweep. resolveMarketplacePriceSchedule's default-schedule branch (used when the caller supplies no explicit price_schedule) silently clamped a negative computed list price to 0 instead of rejecting it:

if listPrice < 0 {
    listPrice = 0
}

A no-upfront RI (upfrontCost <= 0) or one with an unknown/elapsed term (originalTerm <= 0) makes marketplaceResidualPerUnit return 0, so the RI would be listed on AWS Marketplace for free instead of erroring out the way the supplied-schedule branch already does for a non-positive price ("price must be positive").

The sell-consent-modal preview in frontend/src/history.ts had a parallel bug: it computed the preview price with a different formula than the backend (it included recurring monthly_cost, which the backend deliberately excludes since the buyer assumes the recurring obligation post-transfer, and it didn't divide by instance count). This meant the preview could show a nonzero price for exactly the case the backend now rejects, and diverge from the real listing price for multi-count RIs.

Fix

• internal/api/handler_marketplace.go: the default-schedule branch now returns an explicit error ("cannot compute a default listing price for this RI (no upfront cost or unknown term to prorate); supply an explicit price_schedule") instead of silently listing at $0.
• frontend/src/history.ts: the consent-modal preview now mirrors marketplaceResidualPerUnit + resolveMarketplacePriceSchedule's default branch exactly (upfront-only, per-unit, prorated by remaining/original term), and shows "Default list price: unavailable (no upfront cost or unknown term)" instead of a fabricated or zero price when no default can be computed.

Tests

• TestResolveMarketplacePriceSchedule_ZeroDefaultPriceRejected (backend): asserts an error for both upfrontCost=0 and originalTerm=0.
• Two new tests in history-marketplace-sell-button.test.ts (frontend):
  • pins the corrected per-unit formula against the old (wrong) recurring-inclusive row-total formula for a multi-count RI,
  • asserts the no-upfront case shows "unavailable" and never "$0".

Verified both the Go and frontend regression tests fail on the pre-fix code and pass after the fix.

Test plan

• go build ./...
• go vet ./...
• go test ./internal/api/... (full package, includes new test)
• golangci-lint run (v2.10.1, CI-pinned) - 0 issues
• gocyclo -over 10 on changed files - clean
• npx jest src/__tests__/history-marketplace-sell-button.test.ts - 10/10 pass
• npx tsc --noEmit - clean
• npx eslint on changed frontend files - clean
• Confirmed both new regression tests fail on pre-fix code and pass post-fix

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Review details

⚙️ Run configuration

Configuration used: defaults

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Run ID: ac62cdbd-c17c-4b34-87e3-1fd3464acfd8

📥 Commits

Reviewing files that changed from the base of the PR and between f8cdea6 and 99e7c22.

📒 Files selected for processing (4)

• frontend/src/__tests__/history-marketplace-sell-button.test.ts
• frontend/src/history.ts
• internal/api/handler_marketplace.go
• internal/api/handler_marketplace_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/808-zero-price-marketplace-listing

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[cristim]

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