Add NuGet caching and format-check job to CI

[mbroshi-stripe]

Why?

dotnet format and dotnet build/restore re-download NuGet packages on every CI run because there's no package cache configured. This adds actions/cache@v5 to every job that runs dotnet commands, so warm runs skip the network fetch entirely.

There's also no format check in CI today — developers have to remember to run just prepare locally. This adds an explicit format-check job that enforces it on every PR.

What?

• Added actions/cache@v5 for ~/.nuget/packages to all six jobs that run dotnet commands: test, test-net462, build, pack, build-examples, compat. Cache key is keyed on all .csproj files, with an OS prefix so the Linux and Windows jobs maintain separate caches.
• Added a new format-check job (PR-only) that:
  • Uses fetch-depth: 0 so it can diff against the base branch
  • Exits early with success if no .cs files changed
  • When .cs files did change, runs both format commands from just format scoped to only those files via --include, matching the repo's existing format recipe exactly

See Also

N/A

Changelog

• Added NuGet package caching (actions/cache@v5) to all CI jobs that build or restore dotnet packages.
• Added format-check CI job that verifies dotnet format on changed .cs files for every pull request.