fix(package-manager): add missing path field in installability test fixture (#455)

Landrace between #439 (installability tests fixture) and #451 (added
`path: Option<String>` to `TarballResolution`). Both merged green
against their own bases but main's tip won't compile:

    error[E0063]: missing field `path` in initializer of `TarballResolution`
      --> crates/package-manager/src/installability/tests.rs:56:49

Add the missing field with `path: None` so the struct literal matches
the current `TarballResolution` shape.

pacquet/crates/package-manager/src/installability/tests.rs

@@ -57,6 +57,7 @@ fn synthetic_metadata(
             integrity: None,
             tarball: "https://example.test/pkg.tgz".to_string(),
             git_hosted: None,
+            path: None,
         }),
         engines: engines
             .map(|e| e.iter().map(|(k, v)| ((*k).to_string(), (*v).to_string())).collect()),