Support solution filter (.slnf) inputs in dotnet workload restore

[Copilot]

dotnet workload restore did not accept solution filter inputs, even though solution filters are a standard way to scope project graphs in CI/dev workflows. This change aligns workload restore input handling with restore command behavior for .slnf.

• Command input handling

  • Extend dotnet workload restore target-path recognition to treat .slnf as a valid solution input.
  • Ensure the solution filter path is forwarded through the existing restore/MSBuild invocation path without altering workload resolution semantics.

• Consistency with restore command

  • Reuse the same input classification pattern used by dotnet restore so command behavior is consistent across restore surfaces.
  • Preserve existing behavior for .sln/project/directory inputs.

• Focused test coverage

  • Add/update command-level tests to cover .slnf input acceptance and forwarding for workload restore.
  • Keep coverage scoped to the new input type to avoid broad behavioral changes.

Example (now supported):

dotnet workload restore MyRepo.PlatformA.slnf

• Fixes Workload restore - support solution filters #36277

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