CI: Fix of C# tests failures

[jdetter]

Description of Changes

(jdetter): Github marked Ryan's PR as merged because I rebased it without updating the base branch first - my bad.
Original PR: #4120

This PR fixes C#-related CI failures when testing against v1.12.0 before the corresponding NuGet packages are available on nuget.org.

• CI: Ensure local C# NuGet packages are used consistently
  • Pack crates/bindings-csharp/{BSATN.Runtime,Runtime} with -c Release so the configured local package sources (bin/Release) actually contain the .nupkgs.
  • Run ./sdks/csharp/tools~/write-nuget-config.sh in CI to generate a NuGet.Config that maps SpacetimeDB.BSATN.Runtime / SpacetimeDB.Runtime to those local sources (with nuget.org as fallback for non-SpacetimeDB dependencies).
  • Add an explicit dotnet restore --configfile NuGet.Config step before tests, and run dotnet test --no-restore, so restore always uses the intended config and does not “accidentally” restore from nuget.org.
• write-nuget-config.sh: Make NuGet config discovery + mapping deterministic
  • Write NuGet.Config (capitalized) and include <clear /> + an explicit nuget.org source to avoid inherited sources and to keep PackageSourceMapping functional.
  • Also write a repo-root NuGet.Config so dotnet publish/pack invoked from templates (outside sdks/csharp/) still discovers the same override behavior.
• Smoketests quickstart: avoid PackageSourceMapping restore failures
  • Ensure smoketest helper packing uses Release, and when repo-root NuGet.Config exists, perform dotnet restore --configfile ... + dotnet pack --no-restore so pack/restore are evaluated with the same sources/mapping.
  • When editing configs, prefer NuGet.Config casing for Linux discovery and ensure nuget.org exists as a source for the fallback mapping.

API and ABI breaking changes

No changes

Expected complexity level and risk

1

Testing

• Locally tested concept after simulating local repro.
• Confirmed CI passes in this branch

[bfops]

LGTM, it seems to work in a test PR for me! (#4124)