Use structs for JSON serialization (#19270)
## Summary
See https://github.com/astral-sh/ruff/pull/19133#discussion_r2198413586
for recent discussion. This PR moves to using structs for the types in
our JSON output format instead of the `json!` macro.
I didn't rename any of the `message` references because that should be
handled when rebasing #19133 onto this.
My plan for handling the `preview` behavior with the new diagnostics is
to use a wrapper enum. Something like:
```rust
#[derive(Serialize)]
#[serde(untagged)]
pub(crate) enum JsonDiagnostic<'a> {
Old(OldJsonDiagnostic<'a>),
}
#[derive(Serialize)]
pub(crate) struct OldJsonDiagnostic<'a> {
// ...
}
```
Initially I thought I could use a `&dyn Serialize` for the affected
fields, but I see that `Serialize` isn't dyn-compatible in testing this
now.
## Test Plan
Existing tests. One quirk of the new types is that their fields are in
alphabetical order. I guess `json!` sorts the fields alphabetically? The
tests were failing before I sorted the struct fields.
## Other formats
It looks like the `rdjson`, `sarif`, and `gitlab` formats also use
`json!`, so if we decide to merge this, I can do something similar for
those before moving them to the new diagnostic format.
