fix(azure): web path silently coerces partial-upfront to all-upfront (bill schedule mismatch)

[cristim]

Silent-money bug (web/API path)

crossProviderPaymentAlias (internal/config/validation.go:151-153) coerces an Azure partial-upfront payment option to "upfront" before validation, with the comment "coerce to the all-upfront tier so the rec survives validation."

On the web/API purchase path this means:

POST purchase {provider: azure, service: compute, payment: "partial-upfront"}
  -> normalized to "upfront" (internal/api/validation.go:568)
  -> passes the {upfront, monthly} whitelist
  -> rec.Payment = "upfront"
  -> buildReservationBody maps to billingPlan=Upfront
  -> purchased ALL-UPFRONT silently, on a schedule the caller did not request

Why it matters now

PR #1495 wired Azure billingPlan from the payment option. Before that, Azure always billed upfront regardless, so the coercion was invisible. Now the coerced value drives a live billing schedule, so the silent coercion produces a real cash-flow outcome the caller did not choose. It also contradicts the contract the MCP tool and docs advertise ("partial-upfront is a hard error rather than silently purchased under all-upfront or no-upfront").

This is the exact class the project's feedback_no_silent_fallbacks rule targets: a money-affecting field silently rewritten instead of rejected.

Mitigating factors (why should-fix, not blocker)

• The normal web UI only offers converter-generated recommendations (providers/azure/internal/recommendations/converter.go:391,397 emit only upfront/monthly), so a typical user cannot trigger this; it requires a hand-crafted partial-upfront payload.
• Azure charges the same total for upfront vs monthly, so this is a billing-schedule/cash-flow mismatch, not an overcharge.
• Surface inconsistency: the CLI path now fails loud on Azure partial-upfront (errors in buildReservationBody), while the web path coerces. The two surfaces should behave the same.

Fix

Reject Azure partial-upfront at the web/API validation boundary (fail loud, consistent with the CLI and MCP surfaces) instead of aliasing it to upfront. Azure supports only Upfront and Monthly; partial-upfront has no Azure equivalent and must not be silently mapped.

Found by

Adversarial money-path review during PR #1495 (Azure billing-plan wiring). Pre-existing (the alias predates #1495); #1495 is what makes the coercion reach a live billingPlan.