Fix IndexCreationTool exiting with code 0 on signtool/makeappx failure

[Copilot]

📖 Description

IndexCreationTool was silently succeeding (exit code 0) even when signtool.exe or makeappx.exe failed. Helpers.RunCommand launched child processes but never checked their exit codes, so errors like SignTool Error: No certificates were found that met all the given criteria were printed to stdout but swallowed without throwing.

Changes:

• RunCommand — checks p.ExitCode after WaitForExit() and throws if non-zero, causing the error to propagate up through CreatePackage → CreateLocalSource → Main, which returns -1
• Unpack — was inlining its own process-launch logic (also without exit code check); refactored to delegate to RunCommand

🔗 References

🔍 Validation

Verified by code inspection that all signtool/makeappx invocations route through RunCommand, and that Main catches any thrown exception and returns -1.

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated Release Notes (if applicable)
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)

📋 Issue Type

• Bug fix
• Feature
• Task