[red-knot] Add support for untitled files (#12492)

## Summary

This PR adds support for untitled files in the Red Knot project.

Refer to the [design
discussion](https://github.com/astral-sh/ruff/discussions/12336) for
more details.

### Changes
* The `parsed_module` always assumes that the `SystemVirtual` path is of
`PySourceType::Python`.
* For the module resolver, as suggested, I went ahead by adding a new
`SystemOrVendoredPath` enum and renamed `FilePathRef` to
`SystemOrVendoredPathRef` (happy to consider better names here).
* The `file_to_module` query would return if it's a
`FilePath::SystemVirtual` variant because a virtual file doesn't belong
to any module.
* The sync implementation for the system virtual path is basically the
same as that of system path except that it uses the
`virtual_path_metadata`. The reason for this is that the system
(language server) would provide the metadata on whether it still exists
or not and if it exists, the corresponding metadata.

For point (1), VS Code would use `Untitled-1` for Python files and
`Untitled-1.ipynb` for Jupyter Notebooks. We could use this distinction
to determine whether the source type is `Python` or `Ipynb`.

## Test Plan

Added test cases in #12526

crates/ruff_db/src/source.rs

@@ -8,7 +8,7 @@ use ruff_notebook::Notebook;
 use ruff_python_ast::PySourceType;
 use ruff_source_file::LineIndex;
 
-use crate::files::File;
+use crate::files::{File, FilePath};
 use crate::Db;
 
 /// Reads the source text of a python text file (must be valid UTF8) or notebook.
@@ -16,25 +16,33 @@ use crate::Db;
 pub fn source_text(db: &dyn Db, file: File) -> SourceText {
     let _span = tracing::trace_span!("source_text", ?file).entered();
 
-    if let Some(path) = file.path(db).as_system_path() {
-        if path.extension().is_some_and(|extension| {
+    let is_notebook = match file.path(db) {
+        FilePath::System(system) => system.extension().is_some_and(|extension| {
             PySourceType::try_from_extension(extension) == Some(PySourceType::Ipynb)
-        }) {
-            // TODO(micha): Proper error handling and emit a diagnostic. Tackle it together with `source_text`.
-            let notebook = file.read_to_notebook(db).unwrap_or_else(|error| {
-                tracing::error!("Failed to load notebook: {error}");
-                Notebook::empty()
-            });
-
-            return SourceText {
-                inner: Arc::new(SourceTextInner {
-                    kind: SourceTextKind::Notebook(notebook),
-                    count: Count::new(),
-                }),
-            };
+        }),
+        FilePath::SystemVirtual(system_virtual) => {
+            system_virtual.extension().is_some_and(|extension| {
+                PySourceType::try_from_extension(extension) == Some(PySourceType::Ipynb)
+            })
         }
+        FilePath::Vendored(_) => false,
     };
 
+    if is_notebook {
+        // TODO(micha): Proper error handling and emit a diagnostic. Tackle it together with `source_text`.
+        let notebook = file.read_to_notebook(db).unwrap_or_else(|error| {
+            tracing::error!("Failed to load notebook: {error}");
+            Notebook::empty()
+        });
+
+        return SourceText {
+            inner: Arc::new(SourceTextInner {
+                kind: SourceTextKind::Notebook(notebook),
+                count: Count::new(),
+            }),
+        };
+    }
+
     let content = file.read_to_string(db).unwrap_or_else(|error| {
         tracing::error!("Failed to load file: {error}");
         String::default()