Use settings::resolve_preview for project validation (#18447)

## Summary

The affected code was manually implementing part of the full logic which
was already available in `settings::resolve_preview` so this PR just
makes the project validation code use this function instead.

## Test Plan

Existing test coverage.

crates/uv/src/lib.rs

@@ -111,11 +111,11 @@ async fn run(mut cli: Cli) -> Result<ExitStatus> {
 
     if !skip_project_validation {
         if let Some(project_path) = cli.top_level.global_args.project.as_ref() {
-            // Resolve the preview flags until this becomes stabilized. We check CLI args and
-            // the `UV_PREVIEW` env var, but not workspace config (which requires reading from
-            // the project directory that may not exist).
+            // Resolve the preview flags until this becomes stabilized. We do
+            // not pass a workspace configuration as this would require reading
+            // from the project directory which might not exist.
             let preview = Preview::from_args(
-                cli.top_level.global_args.preview || environment.preview.value == Some(true),
+                settings::resolve_preview(&cli.top_level.global_args, None, &environment),
                 cli.top_level.global_args.no_preview,
                 &cli.top_level.global_args.preview_features,
             );

crates/uv/src/settings.rs

@@ -213,7 +213,7 @@ fn resolve_python_preference(
 }
 
 /// Resolve the preview setting from CLI, environment, and workspace config.
-fn resolve_preview(
+pub(crate) fn resolve_preview(
     args: &GlobalArgs,
     workspace: Option<&FilesystemOptions>,
     environment: &EnvironmentOptions,