[TEST] Golden file test infrastructure for JSON Schema baselines

[mojodna]

The JSON Schema baseline tests share identical boilerplate: each test file independently implements golden-file comparison, JSON Schema generation, and baseline update logic.

Extract the shared pattern into a reusable module (tests/golden_support.py) and simplify each baseline test to a declarative call. Add a root conftest.py with a --update-baselines flag and registered baseline marker for selective test runs.