fix: exclude packages without artifacts from PEX lock file

- Add validation to only include packages with at least one artifact
- Skip packages that have no downloadable wheels or source distributions
- Prevents PEX parser error: "expected to have at least one artifact"

This ensures all locked requirements have valid downloadable artifacts
as required by the PEX specification.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

crates/uv-resolver/src/lock/export/pex_lock.rs

@@ -199,6 +199,8 @@ impl PexLock {
             }
 
             if let Some(version) = &package.id.version {
+                // Only include packages that have at least one artifact
+                if !artifacts.is_empty() {
                     // Collect dependencies for this package
                     let mut requires_dists = Vec::new();
                     for dep in &package.dependencies {
@@ -221,6 +223,7 @@ impl PexLock {
                     });
                 }
             }
+        }
 
         let locked_resolves = vec![PexLockedResolve {
             locked_requirements,