check_plugin.py, render_plugin.py, and the pytest plugin matrix each built
config as {"enabled": True} then merged in config_schema.json's defaults on
top, letting a plugin's own enabled:false default (a reasonable choice for
a seasonal/opt-in plugin -- 15 of 23 real plugins ship one) silently win.
Every harness/CI render of those plugins was testing "disabled, do
nothing" rather than real behavior.
Extract build_full_config() into testing/loading.py (already the shared
home for plugin-discovery/config-default logic) and use it from all three
call sites: schema defaults, then a forced enabled=True, then harness.json's
config, then the caller's explicit config -- so a test can still
deliberately disable a plugin on purpose, it just can't happen by accident
via the plugin's own shipped schema default anymore.
