fix(dotnet): explicit_projects must stop at the user's own -- boundary

Dialect::Msbuild keeps classifying tokens past `--` (it's a forwarding
boundary for the test runner, not an end-of-options marker), so a
VSTest/MTP filter expression after -- that coincidentally ends in a
project extension (e.g. `dotnet test -- FullyQualifiedName~Foo.csproj`)
was picked up by explicit_projects as if it were a real project
reference, skipping the working-directory scan and risking the wrong
TestRunnerMode. Confirmed against a real dotnet 9 SDK that this exact
invocation runs the real project in cwd, not a nonexistent project
named after the filter text.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>