fix: Handle null execution_error in job schema

The API returns execution_error: null for jobs without errors,
but the Zod schema only had .optional() which doesn't handle null.
This caused the entire parse to fail, resulting in 0 active jobs.

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

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

src/platform/remote/comfyui/jobs/jobTypes.ts

@@ -55,7 +55,7 @@ const zRawJobListItem = z
     execution_end_time: z.number().nullable().optional(),
     preview_output: zPreviewOutput.nullable().optional(),
     outputs_count: z.number().nullable().optional(),
-    execution_error: zExecutionError.optional(),
+    execution_error: zExecutionError.nullable().optional(),
     workflow_id: z.string().nullable().optional(),
     priority: z.number().optional()
   })