fix: set MAX_PIPELINE_STEPS=0 in test env to match unlimited assumption

Two pipeline tests ("accepts pipeline with more than 20 steps when
limit is unlimited") expected unlimited steps but the test env didn't
set MAX_PIPELINE_STEPS, falling back to the default of 20.

vitest.config.ts

@@ -56,6 +56,7 @@ export default defineConfig({
       CONCURRENT_JOBS: "3",
       FILE_MAX_AGE_HOURS: "1",
       CLEANUP_INTERVAL_MINUTES: "60",
+      MAX_PIPELINE_STEPS: "0",
     },
     coverage: {
       provider: "v8",