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-mslearn/toolkit/changelog.md

@@ -981,6 +981,7 @@ _**Breaking change**_
     - Added param to disable external access to Azure Data Lake and Azure Data Explorer.
     - Added param to specify subnet range of virtual network - minimum size = /26
   - Support for storage account infrastructure encryption.
+  - 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.
   - Published a [schema file](https://aka.ms/finops/hubs/settings-schema) for the hub settings.json file.
 - **Changed**
   - Changed dataset names in the ingestion container to facilitate Azure Data Explorer ingestion.