Add regression test for contentFiles include with leading separator

[donnie-msft]

Bug

Fixes: https://github.com/NuGet/Client.Engineering/issues/3901

Description

A customer reported that in VS 18.8, a contentFiles entry with a leading separator in the include path was silently ignored when restoring in Visual Studio.
e.g.  \any\net10.0\data\* had the configured buildAction and copyToOutput dropped in VS, while dotnet build worked correctly.

In VS 18.9, this was already fixed, but no test was added for the scenario.

Root cause

The content-file globbing introduced in #7338 built match paths as MatcherRoot ("ROOT/" ) + the relative path. When an include starts with a separator, this produced a malformed path (ROOT/\any ) that Microsoft.Extensions.FileSystemGlobbing  failed to match. The entry was dropped and its build action fell back to the default (Compile) silently with no error.

The bug was already fixed by #7405, which reworked matching to use InMemoryDirectoryInfo + Matcher.Execute . However, no test ever covered this scenario, so the fix is unguarded against regression.

What this adds

This test covers leading forward-slash, leading backslash, interior-backslash, and canonical include forms. Each asserts the resolved build action is None (the authored value) rather than the default Compile , proving the glob matched.

Verification

Test passes on dev (has fix from #7405).
Test FAILs on release/7.8.x for the leading-separator cases (Expected: None, Actual: Compile) and this confirms it reproduces the customer regression.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.