Fix export prerequisites and supported region drift

[Deepthi Chelupati (dm-chelupati)]

Summary

• Detect and provision PyYAML before Bash and PowerShell exports begin, with an isolated virtual-environment fallback when user-site installation is unavailable
• Add explicit dependency opt-out flags and actionable prerequisite failure messages
• Expand deployment, clone, recipe, and starter-lab allowlists from 4 regions to all 20 verified live regions, including polandcentral and southcentralus
• Add a canonical supported-region manifest and consistency test to prevent template, recipe, lab, Terraform, Bicep, and documentation drift
• Incorporate and complete the region update proposed in fix: expand supported-regions allowlist to match official list #317

Fixes #300
Related: serverless-paas-balam/sreagent-internal#1452

Validation

• Bash syntax validation passed
• Bash missing-PyYAML, automatic-install, and isolated-environment tests passed
• PowerShell missing-PyYAML behavior test passed
• All 15 PowerShell files parsed with zero errors on PowerShell 7.5.1
• Region consistency test passed for all 20 regions
• Core and starter-lab Bicep compilation passed
• Terraform validation passed
• JSON validation and git diff --check passed
• Stale restrictive region-pattern scan returned no remaining matches