fix(plugins): core's own config keys no longer flag plugins as degraded (#434)

Found while sweeping devpi for issues. Nine of 27 installed plugins were
reported degraded in the web UI -- including baseball-scoreboard and
f1-scoreboard -- for using a documented core feature.

The core reads three tuning keys out of each plugin's own config block:
vegas_width_pct and vegas_overflow (vegas_mode/plugin_adapter.py) and
vegas_max_width_screens (base_plugin.py). No plugin declares them, and 37 of
the 42 published config schemas set "additionalProperties": false -- so schema
validation reported them as violations.

That is not just log noise. _validate_config_schema_soft sets `degraded` in
the health tracker, which the web UI surfaces, so a user who tuned a core
Vegas setting saw the plugin marked broken.

The keys are stripped before validation. Fixing it plugin-side would mean 42
schema edits and 42 version bumps -- 42 store updates for a contract the core
owns.

Listed explicitly rather than matched on a `vegas_` prefix: vegas_mode is the
opposite case, plugin-owned and declared in schemas, and a prefix rule would
silently stop validating it.

Verified on devpi: degraded went 9 of 27 -> 0 of 27, schema-mismatch warnings
9 -> 0, 22 plugins still load, no tracebacks. 800 core unit tests pass,
8 of them new -- including that a genuine violation is still reported, so the
check has not been turned into a no-op, and that the caller's live config dict
is never mutated.


Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/plugin_system/plugin_manager.py

@@ -395,6 +395,37 @@ class PluginManager:
             self.state_manager.set_state(plugin_id, PluginState.ERROR, error=e)
             return False
     
+    #: Config keys the **core** reads out of a plugin's own config block. The
+    #: plugin never declares them, so a schema with
+    #: ``"additionalProperties": false`` — 37 of the 42 published ones — reports
+    #: them as violations and the plugin gets flagged degraded in the web UI for
+    #: using a documented core feature.
+    #:
+    #: Listed explicitly rather than matched on a ``vegas_`` prefix, because
+    #: ``vegas_mode`` is the opposite case: plugins *do* declare that one, and a
+    #: prefix rule would silently stop validating it.
+    #:
+    #: Read by: ``vegas_mode/plugin_adapter.py`` (``vegas_width_pct``,
+    #: ``vegas_overflow``) and ``base_plugin.py`` (``vegas_max_width_screens``).
+    CORE_OWNED_CONFIG_KEYS = frozenset({
+        'vegas_width_pct',
+        'vegas_overflow',
+        'vegas_max_width_screens',
+    })
+
+    def _strip_core_owned_keys(self, config: Dict[str, Any]) -> Dict[str, Any]:
+        """A shallow copy of ``config`` without the core's own tuning keys.
+
+        Only the top level is touched, and only when such a key is present, so
+        the common case allocates nothing extra.
+        """
+        if not isinstance(config, dict):
+            return config
+        if not self.CORE_OWNED_CONFIG_KEYS.intersection(config):
+            return config
+        return {k: v for k, v in config.items()
+                if k not in self.CORE_OWNED_CONFIG_KEYS}
+
     def _validate_config_schema_soft(self, plugin_id: str, config: Dict[str, Any]) -> None:
         """Validate a plugin's config against its JSON schema — warn/degrade only.
 
@@ -419,7 +450,7 @@ class PluginManager:
 
         try:
             is_valid, errors = self.schema_manager.validate_config_against_schema(
-                config, schema, plugin_id
+                self._strip_core_owned_keys(config), schema, plugin_id
             )
         except Exception as e:  # pragma: no cover - defensive
             # Validation machinery itself failed — do not penalise the plugin.