fix: preserve ISO builder decoding in Windows PowerShell

[Timpan4]

A real ISO build under Windows PowerShell 5.1 failed before validation because build-iso.ps1 contains UTF-8 characters without a BOM. Native decoding changed the parsed structure and executed catch as a command. Add the UTF-8 BOM and a regression that invokes the Windows PowerShell parser and verifies the main try/catch/finally block.

Validation:

• Focused Windows PowerShell 5.1 parser regression passed.
• Independent requested review found no actionable defects; git diff --check passed.
• Installed Microsoft-signed ADK Deployment Tools 10.1.26100.2454 and all six applicable KB5101684 patches on a host with no previous Deployment Tools installation; all installers returned 0.
• Elevated Windows PowerShell 5.1 build using Windows 11 25H2 x64 media passed Windows SIM schema validation, indexed metadata validation of all 11 editions, staging validation, and oscdimg generation with exit 0.
• Generated ISO is 8,454,518,784 bytes. Remounted the generated ISO and verified its required setup payload and unattend references. Used apps-template.json as the disposable apps.json; no personal configuration was copied.

This completes the real build and freshly installed prerequisite validation left open by #107. No Windows installation or boot test was performed or required by these issues.

Closes #58
Closes #59

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 7c9b29b and e6937c4.

📒 Files selected for processing (2)

• build-iso.ps1
• tests/BuildIso.Tests.ps1

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