Add an explicit solution-level per-project `Platform` for `.csproj`
projects even if it is the same as the solution-wide `Platform`.
Otherwise the VS IDE skips building it.
Fixes: #27330
Refactoring in commit 3882718872 (VS: Decouple solution generation from
`.sln` file format, 2025-09-15) accidentally left out automatic
detection of the external project type id from its file extension.
Since the `Visual Studio 18 2026` generator is new, we can switch
to the `.slnx` file format without changing behavior for users of
VS 2022 and older.
Fixes: #25887
Brad King