handle errant whitespace in global.json

[brettfo]

The JSON parser in PowerShell doesn't like leading newline characters which makes handling global.json files difficult. This PR forces the file to be read as a single string with -Raw which enables ConvertFrom-Json to work.

[Copilot]

Pull request overview

Updates the NuGet updater’s PowerShell logic for reading global.json so SDK version discovery is resilient to leading whitespace/newlines, and adds regression test scenarios to validate the behavior.

Changes:

• Read global.json using Get-Content -Raw before ConvertFrom-Json to avoid line-by-line parsing edge cases.
• Add a new Test-GlobalJsonVersions scenario covering leading newline/whitespace (and a leading comment case).
• Add new global.json fixtures under nuget/updater/test-data/global-json-whitespace/.

Show a summary per file

File	Description
nuget/updater/common.ps1	Switches global.json reading to Get-Content -Raw before JSON conversion.
nuget/updater/test.ps1	Adds a regression test case for global.json files with leading whitespace/newlines.
nuget/updater/test-data/global-json-whitespace/leading-whitespace/global.json	Fixture: global.json starting with indentation before {.
nuget/updater/test-data/global-json-whitespace/leading-newline/global.json	Fixture: global.json starting with a blank line.
nuget/updater/test-data/global-json-whitespace/leading-comment/global.json	Fixture: global.json starting with a // comment line.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 0