fix(testing): stop plugin enabled:false schema defaults from silently disabling harness tests (#408)

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.

test/plugins/test_plugin_matrix.py

@@ -22,7 +22,7 @@ import pytest
 from src.plugin_system.testing.harness import (
     render_plugin_matrix, compare_to_goldens,
 )
-from src.plugin_system.testing.loading import load_config_defaults, load_harness_spec
+from src.plugin_system.testing.loading import build_full_config, load_harness_spec
 from src.plugin_system.testing.sizes import resolve_test_sizes
 
 PROJECT_ROOT = Path(__file__).resolve().parents[2]
@@ -81,9 +81,7 @@ def test_plugin_renders_across_sizes_and_screens(plugin_id: str) -> None:
     plugin_dir = _PLUGINS[plugin_id]
     spec = load_harness_spec(plugin_dir)
 
-    config = {"enabled": True}
-    config.update(load_config_defaults(plugin_dir))
-    config.update(spec.get("config", {}))
+    config = build_full_config(plugin_dir, spec)
 
     # Sizes: LEDMATRIX_TEST_SIZES env (test on real hardware) wins, then the
     # plugin's own harness.json "sizes", else the default representative sample.