fix: address PR microsoft#2006 review feedback

Comprehensive rewrite of Generate-MultiCloudTestData.ps1:

Critical fixes:
- Fix Get-Random [int] overflow with 12-digit AWS account IDs (New-AwsAccountId)
- Eliminate Python dependency entirely (inline budget scaling via scale factor)
- Remove dead code from Python/Parquet block

Required by repo conventions:
- Add #Requires -Version 7.0
- Add .LINK to comment-based help
- Add [CmdletBinding(SupportsShouldProcess)] with WhatIf/Confirm support
- Add changelog entry
- Add test directory README.md

FOCUS specification compliance:
- Fix ~12 ServiceSubcategory values to match FOCUS closed enumeration
- Fix cost invariants: unit prices calculated AFTER all cost modifications
- Anomaly rows now set ChargeClass=Correction (exempt from invariant rules)
- Credits/Adjustments get null InvoiceId (per FOCUS spec)
- Version-aware column sets: v1.1+ gets CommitmentDiscountQuantity/Unit,
  v1.2+ gets BillingAccountType/SubAccountType/InvoiceId,
  v1.3+ gets HostProviderName/ServiceProviderName
- Document scope as Cost and Usage dataset only

Recommended improvements:
- Add -Seed parameter for reproducible test data
- Add -UseStorageKey switch, default to Azure AD auth (--auth-mode login)
- Fix Get-RandomDecimal to use [long] instead of [int] for large ranges

Author: Zach Olinske

docs/_resources/changelog.md

@@ -72,6 +72,7 @@ Legend:
 >    - Added param to disable external access to Azure Data Lake and Azure Data Explorer.
 >    - Added param to specify subnet range of vnet - minumum size = /26
 > 6. Infrastructure encryption - Added an optional enableInfrastructureEncryption template parameter to support storage account infrastructure encryption.
+> 7. Multi-cloud test data generator – Added `Generate-MultiCloudTestData.ps1` to create synthetic FOCUS-compliant cost data for Azure, AWS, GCP, and on-premises providers. Supports FOCUS versions 1.0-1.3 with version-specific column sets, commitment discounts, Azure Hybrid Benefit, tag variation, and inline budget scaling. Includes `-Seed` for reproducibility and Azure AD auth for uploads.
 
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