Add .slnx solution format support

[mrt181]

Adds .slnx support to OmniSharp. Closes #2699.

.slnx is the default solution format in .NET 10+. Tools embedding OmniSharp (e.g. sonarlint-language-server) still use an embedded build that only understands .sln — they work fine with .sln in .NET 10 but fail on .slnx. This unblocks them.

Uses Microsoft.VisualStudio.SolutionPersistence to parse both .sln and .slnx through SolutionSerializers.GetSerializerByMoniker(), replacing the hand-rolled global section parsing with a single unified code path.

Testing locally with installed net10.0 sdk

AbstractMSBuildTestFixture reads OMNISHARP_TEST_SDK_PATH env var to override MSBuild SDK discovery. Pass it to dotnet test:

dotnet test tests/OmniSharp.MSBuild.Tests/OmniSharp.MSBuild.Tests.csproj -f net10.0 \
  --filter "FullyQualifiedName~WorkspaceInformationTests" \
  --environment OMNISHARP_TEST_SDK_PATH=/path/to/sdk/6.0.428

[mrt181]

@microsoft-github-policy-service agree

[OoLunar]

What's currently blocking this PR from being merged? I'm unable to work with .NET 10 without this. A temporary workaround for this is by using the sln file format instead of slnx, which can be created by executing dotnet new sln --format sln through the .NET CLI.