proposal: Expand test coverage for generator, RPC, and client boundaries

[alsi-lawr]

Add focused tests around metadata parsing, type mapping, generated output, MsgPack/RPC transport behaviour, and client API boundaries.

Motivation

The generator/client refactor will touch high-risk areas: type mapping, MsgPack response decoding, API shape, generated source emission, and RPC failure handling. These need coverage before larger rewrites.

Scope

• Add unit tests for Neovim metadata mapping.
• Add unit tests for type mapping.
• Add tests for identifier naming policy if appropriate.
• Add tests for MsgPack request/response encoding and decoding. This is more useful for when the rpc client is productised, as a richer IO boundary would be a perfect enhancement here.
• Add golden-file tests for generated C# output (very lean coverage).

Acceptance criteria

• Type mapping has meaningful unit coverage.
• Identifier naming has meaningful unit coverage (if appropriate).
• Generated output can be verified with golden files.
• Integration tests are clearly separated from unit tests.
• Tests fail clearly on decode/transport/protocol errors.

Blocked by

• proposal: Add enforced CSharpier formatting #27
• proposal: Remove vague utility types and migrate to System.Text.Json #28
• proposal: Rework test boundaries and migrate tests to xUnit/Shouldly #29

[justinmk]

The code cost here should be less than 300 lines, otherwise we're doing something wrong. This repo should not require huge amounts of code or tests. It should be very simple.