Make publishDate optional in output schema

The Zod schema used z.union([z.string(), z.undefined()]) which requires
the key to be present. Providers like Mise never set publishDate, so
the key is absent entirely. Changed to z.string().optional() which
allows missing keys.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: stevelandeydescript

src/core/schema.ts

@@ -6,7 +6,7 @@ const dependencyVersionSchema = z.object({
     latestVersion: z.string(),
     usedBy: z.array(z.string()),
     dependencyTypes: z.array(z.enum(['dev', 'prod'])),
-    publishDate: z.union([z.string(), z.undefined()]),
+    publishDate: z.string().optional(),
     inCatalog: z.boolean(),
 });