Improve error bodies and move to release-please with NuGet OIDC

[perkops]

Summary

• Read error bodies as text whatever their media type
• Add typed error content, content type and headers to binary responses
• Decode non-ASCII download file names via RFC 5987 filename*
• Replace GitFlow promotion and NBGV with release-please
• Publish to NuGet via Trusted Publishing (OIDC) instead of an API key

Changes

✨ Features

• Add ErrorContentObject deserialized via AddErrorResponse
• Fill ContentType and ContentLength on binary error responses
• Expose response and content Headers on binary responses
• Prefer Content-Disposition filename* over filename for FileName
• Add new members via a constructor overload; interfaces unchanged

🐛 Fixes

• Read error bodies with non-text media types as a string
• Decode error bodies by the Content-Type charset, UTF-8 by default
• Keep byte[] handling for non-text success bodies

👷 CI/CD

• Add release-please workflow publishing to NuGet via NuGet/login OIDC
• Rename pre-integration workflow to build and run it on main pushes
• Pin GitHub Actions to commit SHAs
• Remove post-integration and manual release workflows

🔧 Configuration

• Add release-please config and manifest for a single package
• Replace Nerdbank.GitVersioning with a Version in Directory.Build.props
• Ignore the local issues folder

📝 Documentation

• Document binary error handling, file names and 3xx behavior
• Document error bodies with non-text media types in the README

🔥 Removal

• Remove version.json and the retired GitFlow docs and images