Unify OldDiagnostic and Message (#18391)

Summary
--

This PR unifies the remaining differences between `OldDiagnostic` and
`Message` (`OldDiagnostic` was only missing an optional `noqa_offset`
field) and
replaces `Message` with `OldDiagnostic`.

The biggest functional difference is that the combined `OldDiagnostic`
kind no
longer implements `AsRule` for an infallible conversion to `Rule`. This
was
pretty easy to work around with `is_some_and` and `is_none_or` in the
few places
it was needed. In `LintContext::report_diagnostic_if_enabled` we can
just use
the new `Violation::rule` method, which takes care of most cases.

Most of the interesting changes are in [this
range](8156992540)
before I started renaming.

Test Plan
--

Existing tests

Future Work
--

I think it's time to start shifting some of these fields to the new
`Diagnostic`
kind. I believe we want `Fix` for sure, but I'm less sure about the
others. We
may want to keep a thin wrapper type here anyway to implement a `rule`
method,
so we could leave some of these fields on that too.

crates/ruff_server/src/lint.rs

@@ -15,7 +15,7 @@ use ruff_linter::{
     directives::{Flags, extract_directives},
     generate_noqa_edits,
     linter::check_path,
-    message::Message,
+    message::OldDiagnostic,
     package::PackageRoot,
     packaging::detect_package_root,
     settings::flags,
@@ -122,7 +122,7 @@ pub(crate) fn check(
     let directives = extract_directives(parsed.tokens(), Flags::all(), &locator, &indexer);
 
     // Generate checks.
-    let messages = check_path(
+    let diagnostics = check_path(
         &query.virtual_file_path(),
         package,
         &locator,
@@ -139,7 +139,7 @@ pub(crate) fn check(
 
     let noqa_edits = generate_noqa_edits(
         &query.virtual_file_path(),
-        &messages,
+        &diagnostics,
         &locator,
         indexer.comment_ranges(),
         &settings.linter.external,
@@ -162,7 +162,7 @@ pub(crate) fn check(
     }
 
     let lsp_diagnostics =
-        messages
+        diagnostics
             .into_iter()
             .zip(noqa_edits)
             .filter_map(|(message, noqa_edit)| {
@@ -231,7 +231,7 @@ pub(crate) fn fixes_for_diagnostics(
 /// Generates an LSP diagnostic with an associated cell index for the diagnostic to go in.
 /// If the source kind is a text document, the cell index will always be `0`.
 fn to_lsp_diagnostic(
-    diagnostic: &Message,
+    diagnostic: &OldDiagnostic,
     noqa_edit: Option<Edit>,
     source_kind: &SourceKind,
     index: &LineIndex,