CI: PR test gate + NuGet publishing

[renansj]

Summary

• Add CI workflow that runs on every PR to main: builds and runs all tests with a real Redis service. PR can only merge if all tests pass.
• Add Release workflow that runs on push to main (PR merge): runs tests, then publishes stable packages to nuget.org (default VS feed) and GitHub Packages.
• On PRs, packs a unique prerelease (1.0.0-pr.<n>.<run>) and publishes to GitHub Packages (no version collisions).
• Add Directory.Build.props with shared package metadata (repo URL, MIT license).

Notes

• Two packages published: AppRateLimiter and AppRateLimiter.Redis.
• Sample.NetFramework (net472) is excluded from the Linux CI build (cannot build on Linux).
• NUGET_API_KEY is stored as a repo Actions secret; never in source.

Tested

• First run of this PR exercises the CI gate itself.