## Summary
This PR deletes the `DiagnosticKind` type by inlining its three fields
(`name`, `body`, and `suggestion`) into three other diagnostic types:
`Diagnostic`, `DiagnosticMessage`, and `CacheMessage`.
Instead of deferring to an internal `DiagnosticKind`, both `Diagnostic`
and `DiagnosticMessage` now have their own macro-generated `AsRule`
implementations.
This should make both https://github.com/astral-sh/ruff/pull/18051 and
another follow-up PR changing the type of `name` on `CacheMessage`
easier since its type will be able to change separately from
`Diagnostic` and `DiagnosticMessage`.
## Test Plan
Existing tests
e2c5b83fe1