Merge latest Library.Template

[AArnott]

• Drop SupportsShouldProcess from Get-ExternalSymbolFiles.ps1
• Use dotnet msbuild for -getItem parameters
• Use -LiteralPath where appropriate
• Use Join-Path
• Add missing dotnet symbol command to manifest
• Backport fixes from microbuild branch
• Update dependency nerdbank.dotnetrepotools to v1.3.22 (509)
• Switch AI-assisted merging from a prompt to a skill
• Update dependency powershell to v7.6.1 (510)
• Update Dockerfile and global.json updates to v10.0.203 (511)
• Use safer ToLower variant for package IDs
• Update NuGet/login action to v1.2.0 (512)
• Ignore C# Dev Kit generated cache files
• Update Microsoft Testing Platform to 2.2.2 (514)
• Update dependency nerdbank.dotnetrepotools to v1.4.1 (515)
• Update Dockerfile and global.json updates to v10.0.300 (516)

[Copilot]

Pull request overview

Updates this repo to align with the latest Library.Template, including dependency/tooling version bumps, PowerShell script hardening, and the migration of AI-assisted template merging guidance from a “prompt” artifact to a GitHub skill.

Changes:

• Update SDK/tooling/dependency versions (global.json, Microsoft Testing Platform, dotnet tools).
• Harden/standardize PowerShell scripts (use -LiteralPath, Join-Path, ToLowerInvariant(), and dotnet msbuild for -getItem).
• Add template merge “skill” documentation and ignore C# Dev Kit cache artifacts.

Reviewed changes

Copilot reviewed 8 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tools/Get-NuGetTool.ps1	Uses -LiteralPath when validating nuget.exe path existence for safer path handling.
tools/Get-ExternalSymbolFiles.ps1	Removes SupportsShouldProcess and switches package version evaluation to dotnet msbuild -getItem.
tools/Download-NuGetPackage.ps1	Uses ToLowerInvariant() for stable, culture-invariant package ID normalization.
tools/Convert-PDB.ps1	Uses Join-Path for building the Pdb2Pdb tool path.
global.json	Updates .NET SDK version to 10.0.300.
Directory.Packages.props	Bumps Microsoft Testing Platform version to 2.2.2.
.gitignore	Ignores C# Dev Kit cache files (*.lscache).
.github/skills/update-library-template/template-release-notes.md	Adds template release notes to guide future merges.
.github/skills/update-library-template/SKILL.md	Defines the update-library-template skill and updates merge instructions/reference link.
.config/dotnet-tools.json	Updates dotnet tool versions (PowerShell, nerdbank.dotnetrepotools).