test: lower the count floor for the extensions that left the tree

The cutover removes four builtin extensions, so collection legitimately
drops. Verified against dev by diffing collected node ids rather than
trusting the delta:

  -7  test_extension_manifests  the 7 plugin.json files of the four
                                departing extensions (builtin + frontend)
  -4  test_builtin_extension_drift  anchor swap, email (7 py files) ->
                                    status (3)
   0  test_files_rbac / *_extraction  param-id churn only

No feature lost its proving tests. 4799 -> 4791.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/tests/BASELINE_COUNT

@@ -1 +1 @@
-4799
+4791