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.

scripts/check_plugin.py

@@ -37,7 +37,7 @@ os.environ['EMULATOR'] = 'true'
 
 from src.logging_config import get_logger  # noqa: E402
 from src.plugin_system.testing.loading import (  # noqa: E402
-    find_plugin_dir, load_config_defaults, load_harness_spec, load_manifest,
+    build_full_config, find_plugin_dir, load_harness_spec, load_manifest,
 )
 from src.plugin_system.testing.harness import (  # noqa: E402
     RenderResult, render_plugin_matrix, compare_to_goldens, write_goldens,
@@ -97,12 +97,11 @@ def check_one(plugin_id: str, search_dirs: List[str], sizes, mock_data: Dict,
     # matrix path does; explicit CLI flags still override the file.
     spec = load_harness_spec(plugin_dir)
 
-    # config_schema defaults (real-install behavior), then harness.json config,
-    # then CLI --config — most specific wins.
-    full_config = {"enabled": True}
-    full_config.update(load_config_defaults(plugin_dir))
-    full_config.update(spec.get("config", {}))
-    full_config.update(config)
+    # config_schema defaults (real-install behavior, with enabled forced True
+    # so a plugin's own enabled:false default can't accidentally disable
+    # testing), then harness.json config, then CLI --config — most specific
+    # wins.
+    full_config = build_full_config(plugin_dir, spec, config)
 
     # Precedence: CLI flag > LEDMATRIX_TEST_SIZES env > harness.json > default.
     effective_sizes = sizes if sizes else resolve_test_sizes(spec.get("sizes"))

scripts/render_plugin.py

@@ -28,7 +28,7 @@ os.environ['EMULATOR'] = 'true'
 # Import logger after path setup so src.logging_config is importable
 from src.logging_config import get_logger  # noqa: E402
 from src.plugin_system.testing.loading import (  # noqa: E402
-    find_plugin_dir, load_manifest, load_config_defaults,
+    build_full_config, find_plugin_dir, load_manifest,
 )
 logger = get_logger("[Render Plugin]")
 
@@ -83,16 +83,13 @@ def main() -> int:
     manifest = load_manifest(Path(plugin_dir))
 
     # Parse config: start with schema defaults, then apply overrides
-    config_defaults = load_config_defaults(Path(plugin_dir))
     try:
         user_config = json.loads(args.config)
     except json.JSONDecodeError as e:
         logger.error("Invalid JSON config: %s", e)
         return 1
 
-    config = {'enabled': True}
-    config.update(config_defaults)
-    config.update(user_config)
+    config = build_full_config(Path(plugin_dir), cli_config=user_config)
 
     # Load mock data if provided
     mock_data = {}