Sidecar solution restore broken: MessagePack 3.1.7/3.1.8 downgrade + FSharp.Core 10.1.301 violates FCS exact pin

dotnet restore sidecars/SharpLsp.Sidecars.sln currently fails:

• NU1605 (×2): SharpLsp.Sidecar.CSharp and SharpLsp.Sidecar.CSharp.Tests pin MessagePack 3.1.7, but they reference SharpLsp.Sidecar.Common, which pins 3.1.8 — a package downgrade along a project-reference edge, elevated to error.
• NU1608 (×2): SharpLsp.Sidecar.FSharp and SharpLsp.Sidecar.FSharp.Tests pin FSharp.Core 10.1.301, but FSharp.Compiler.Service 43.12.204 requires exactly FSharp.Core 10.1.204. 43.12.204 is the latest stable FCS on NuGet, so there is no forward bump available — the FSharp.Core pin must match FCS.

Root cause: a partial dependency bump updated MessagePack in Common/FSharp and FSharp.Core in the F# projects without propagating to the C# projects or checking the FCS exact-pin constraint. This blocks building/testing the full sidecar solution.

Fix: align MessagePack to 3.1.8 everywhere; pin FSharp.Core to 10.1.204 (matched pair with FCS 43.12.204). Regression guard: static dependency-consistency tests that fail on any project-reference downgrade (NU1605) and on FSharp.Core/FCS pair drift (NU1608).

Start by inspecting the project files under sidecars/SharpLsp.Sidecars.sln and run dotnet restore sidecars/SharpLsp.Sidecars.sln to reproduce NU1605 and NU1608. Align MessagePack across the C# projects and FSharp.Core with FSharp.Compiler.Service, then add the described static dependency-consistency regression tests and verify restore and tests succeed.