feat(purchases): idempotent re-drive across Azure and GCP (closes #639)

[cristim]

Summary

• Replace allRecsAWS predicate in RecoverStrandedApprovals with allRecsSafeToRedrive + recIsSafeToRedrive, extending idempotent re-drive from AWS-only to Azure reservations and GCP compute.
• Azure reservation clients (compute, database, cache, cosmosdb, managedredis, search, synapse) all call DoIdempotentPurchaseTwoStep with opts.IdempotencyToken (PR fix(providers/azure): restore IdempotencyToken threading in DoPurchaseTwoStep #729 / issue bug(providers/azure): DoPurchaseTwoStep dropped IdempotencyToken threading; reintroduces double-purchase risk (regression of #641; blocks #639) #721), making re-drive safe.
• GCP compute CUDs use server-side RequestId + deterministic commitment name from the token (issue feat(purchases): make GCP Compute commitment creation idempotent (remaining slice of #641) #654), making re-drive safe.
• Azure Savings Plans keeps the safe-fail path: the OrderAlias API uses time.Now().UnixNano() as the alias name with no server-side idempotency key; a re-drive would create a duplicate savings plan.

Idempotency mechanism per provider

Provider	Service	Mechanism	Re-drive
AWS	all	tag-guard or ClientToken (#636/#638)	enabled
Azure	reservations (compute, db, cache, cosmosdb, managedredis, search, synapse)	DoIdempotentPurchaseTwoStep tag-based lookup (#729)	enabled
Azure	savings-plans	timestamp-based alias name, no deduplication	safe-fail preserved
GCP	compute (CUDs)	server-side RequestId + deterministic name (#654)	enabled

Test plan

• Updated three existing Azure safe-fail tests to use savingsplans service (remaining unsafe case)
• Added TestManager_RecoverStrandedApprovals_AzureReservationRedrives -- Azure compute reservation re-drive reaches PurchaseCommitment exactly once, row ends completed
• Added TestManager_RecoverStrandedApprovals_GCPRedrives -- GCP compute CUD re-drive reaches PurchaseCommitment exactly once, row ends completed
• go build ./... clean
• go test github.com/LeanerCloud/CUDly/internal/purchase/... -- 191 passed

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ℹ️ Review info

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Reviewing files that changed from the base of the PR and between 4956d66 and 97b2a72.

📒 Files selected for processing (2)

• internal/purchase/manager.go
• internal/purchase/manager_test.go

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• Create PR with unit tests
• Commit unit tests in branch fix/639-wave7

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