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_wasm/src/lib.rs

@@ -188,7 +188,7 @@ impl Workspace {
         );
 
         // Generate checks.
-        let messages = check_path(
+        let diagnostics = check_path(
             Path::new("<filename>"),
             None,
             &locator,
@@ -205,7 +205,7 @@ impl Workspace {
 
         let source_code = locator.to_source_code();
 
-        let messages: Vec<ExpandedMessage> = messages
+        let messages: Vec<ExpandedMessage> = diagnostics
             .into_iter()
             .map(|msg| ExpandedMessage {
                 code: msg.noqa_code().map(|code| code.to_string()),