ci: restrict the test workflow's GITHUB_TOKEN to contents:read

CodeQL flagged the new unit-tests job for running with the default
unrestricted token; the pre-existing job had the same exposure. Both
jobs only check out the repo and run pytest, so a workflow-level
contents:read is sufficient.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FgbA8SMutQQpXkMG8LMmC4

.github/workflows/test.yml

@@ -5,6 +5,10 @@ on:
   push:
     branches: [main]
 
+# Both jobs only check out the repo and run pytest.
+permissions:
+  contents: read
+
 jobs:
   plugin-safety:
     name: Plugin safety harness + unit tests