fix(linter): downgrade needs optional:true missing-job to warning

parseNeedJobNames is replaced by parseNeedEntries which preserves the
optional flag from each needs: entry. When a referenced job does not
exist and optional:true is set, the finding is now WARNING instead of
ERROR, matching GitLab CI runtime behavior (the dependency is silently
skipped when the job is absent from a conditional include).

Optional missing deps are also excluded from the cycle-detection graph
since there is no real dependency edge to trace.

Adds a fixture case in testdata/needs.yml to prevent regression.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

testdata/needs.yml

@@ -38,3 +38,12 @@ missing-needs-job:
     - echo "bad"
   needs:
     - nonexistent-job    # ERROR: job doesn't exist
+
+# optional: true — missing dep should be WARNING not ERROR
+optional-needs-job:
+  stage: build
+  script:
+    - echo "I depend on something that may not exist"
+  needs:
+    - job: nonexistent-optional-job
+      optional: true