[internal] Return Messages from check_path (#16837)

Summary
--

This PR updates `check_path` in the `ruff_linter` crate to return a
`Vec<Message>` instead of a `Vec<Diagnostic>`. The main motivation for
this is to make it easier to convert semantic syntax errors directly
into `Message`s rather than `Diagnostic`s in #16106. However, this also
has the benefit of keeping the preview check on unsupported syntax
errors in `check_path`, as suggested in
https://github.com/astral-sh/ruff/pull/16429#discussion_r1974748024.

All of the interesting changes are in the first commit. The second
commit just renames variables like `diagnostics` to `messages`, and the
third commit is a tiny import fix.

I also updated the `ExpandedMessage::location` field name, which caused
a few extra commits tidying up the playground code. I thought it was
nicely symmetric with `end_location`, but I'm happy to revert that too.

Test Plan
--

Existing tests. I also tested the playground and server manually.

crates/ruff_linter/src/message/mod.rs

@@ -41,24 +41,25 @@ mod text;
 
 /// Message represents either a diagnostic message corresponding to a rule violation or a syntax
 /// error message raised by the parser.
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq)]
 pub enum Message {
     Diagnostic(DiagnosticMessage),
     SyntaxError(SyntaxErrorMessage),
 }
 
 /// A diagnostic message corresponding to a rule violation.
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq)]
 pub struct DiagnosticMessage {
     pub kind: DiagnosticKind,
     pub range: TextRange,
     pub fix: Option<Fix>,
+    pub parent: Option<TextSize>,
     pub file: SourceFile,
     pub noqa_offset: TextSize,
 }
 
 /// A syntax error message raised by the parser.
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq)]
 pub struct SyntaxErrorMessage {
     pub message: String,
     pub range: TextRange,
@@ -91,6 +92,7 @@ impl Message {
             range: diagnostic.range(),
             kind: diagnostic.kind,
             fix: diagnostic.fix,
+            parent: diagnostic.parent,
             file,
             noqa_offset,
         })
@@ -140,6 +142,18 @@ impl Message {
         }
     }
 
+    pub fn into_diagnostic_message(self) -> Option<DiagnosticMessage> {
+        match self {
+            Message::Diagnostic(m) => Some(m),
+            Message::SyntaxError(_) => None,
+        }
+    }
+
+    /// Returns `true` if `self` is a diagnostic message.
+    pub const fn is_diagnostic_message(&self) -> bool {
+        matches!(self, Message::Diagnostic(_))
+    }
+
     /// Returns `true` if `self` is a syntax error message.
     pub const fn is_syntax_error(&self) -> bool {
         matches!(self, Message::SyntaxError(_))