Introduce csharpier as the project-wide opinionated formatter and enforce it in CI.

Motivation

The project currently has formatting/style churn that makes larger refactors harder to review. Csharpier gives the repo a single formatting source of truth and removes style debates from later PRs. Csharpier also have a nuget package for programmatic c# formatting, so it's a perfect drop-in for the generated RPC client to get automatically formatted (it will plug-in with a very small change).

Scope

• Add csharpier as a local tool using the default csharpier config.
• Format the current codebase.
• Add CI check for formatting.
• Add contributor docs for running the formatter locally.
• Ensure generated code can also be formatted using CSharpier.

Acceptance criteria

• dotnet csharpier --check . or equivalent runs in CI.
• Existing source is formatted.
• README/contributor docs include the local format command.
• Absolutely no behavioural code changes are included.