bug(providers/azure): DoPurchaseTwoStep dropped IdempotencyToken threading; reintroduces double-purchase risk (regression of #641; blocks #639)

[cristim]

Regression discovered while investigating #639

PR #653 (merged) made Azure reservation purchases idempotent via deterministic reservationOrderId derived from opts.IdempotencyToken. PR #680 (commit 65ecbf81c, merged after #653) then switched all 7 Azure service executors (cache, compute, cosmosdb, database, search, managedredis, synapse) to reservations.DoPurchaseTwoStep (providers/azure/services/internal/reservations/purchase.go:106). This two-step flow:

1. Calls calculatePrice and Azure mints a fresh server-side reservationOrderId on every call.
2. Calls purchase with that fresh ID.

opts.IdempotencyToken is no longer threaded into either step. A second purchase attempt for the same (executionID, recIndex) produces a different reservationOrderId → Azure has no way to dedupe → double-purchase risk.

PR #680's commit body promised "caller-level deduplication uses the purchase-automation tag", but the caller-level guard was never implemented:

• internal/purchase/execution.go:710 executeSinglePurchase calls PurchaseCommitment directly without any pre-flight lookup.
• The only tag attached is cudly-purchase-source: web|cli — not unique per execution, so it can't serve as a dedupe key.

Why it matters now

Issue #639 (auto-re-drive on stranded approvals, unblocked by #641 closing) cannot proceed safely. The whole point of #641 was to make re-drive double-purchase-proof; #680 silently broke that invariant for Azure after #641 was already closed. A stranded Azure approval re-driven by the proposed #639 flip would create a second Azure reservation for every rec whose original attempt reached Azure but didn't save the row — exactly the scenario #639 is meant to make safer.

Fix direction

Either:

1. Restore client-side idempotency in DoPurchaseTwoStep: derive the order ID deterministically from opts.IdempotencyToken (the pattern PR feat(purchases): make Azure reservation purchases idempotent (refs #641) #653 used pre-fix(azure): switch 7 reservation clients to two-step calculatePrice->purchase flow (closes #677) #680); pass it into the two-step flow so the second step reuses the existing ID.
2. Implement the missing caller-level guard: before calling PurchaseCommitment, look up any existing reservation tagged with the deterministic per-execution token; if found, short-circuit (mirroring the AWS EC2 findRIByIdempotencyToken pattern in providers/aws/services/ec2/client.go:126-137). Tag every purchase with that token post-create.

Option 1 is cleaner if Azure supports client-supplied order IDs through the two-step flow. Option 2 mirrors the proven AWS EC2 pattern when client-supplied IDs aren't supported. Investigate which Azure actually accepts.

Tests required

• A re-drive with the same IdempotencyToken on every Azure service does NOT create a second reservation (mock-asserted: either second call short-circuits, or Azure dedupes server-side because the order ID is identical).
• The fix preserves PR fix(azure): switch 7 reservation clients to two-step calculatePrice->purchase flow (closes #677) #680's two-step flow benefits.

Blocking

This is a P1 blocker for #639 and a partial regression of #641. Mark priority/p1, severity/high — the financial-correctness invariant from #641 is broken.

Source of finding

implement-639 investigation comment: #639 (comment)

[cristim]

Verified shipped: PRs #728 (commit 2089692, partial #632) and #729 (commit 6fcf6a6, merged 2026-05-26) are both in feat/multicloud-web-frontend. PR #729 restores IdempotencyToken threading in DoPurchaseTwoStep for all 7 Azure executors; tests added in purchase_test.go and compute/client_test.go.