[api] Populate schema defaults for transitive cpp test deps; add json override

- script/build_helpers.py: when injecting a non-MULTI_CONF component
  into the post-validation config, run its CONFIG_SCHEMA with {} so
  defaults are populated. Without this, socket got config = {} and
  socket.FILTER_SOURCE_FILES crashed with KeyError on
  'implementation' (the schema's defaulted key was never filled in).
  Falls back to {} if the schema can't validate empty input.

- tests/components/json/__init__.py: enable codegen for json so its
  to_code runs during cpp unit test builds, registering the
  ArduinoJson library. Required for any api dep test, since
  json_util.cpp #includes <ArduinoJson.h>.

Locally verified 'script/cpp_unit_test.py api' now compiles and runs;
ProtoMacVarint test suite (9 cases) passes.

script/build_helpers.py

@@ -326,11 +326,21 @@ def compile_and_get_binary(
         # system, not a real loadable component (get_component returns None)
         elif (component := get_component(component_name)) is not None:
             # MULTI_CONF components store their config as a list of dicts,
-            # everything else stores a single dict. Using the wrong shape
-            # breaks code paths that subscript CORE.config[component] with
-            # a string key (e.g. socket.FILTER_SOURCE_FILES).
-            default = [] if component.multi_conf else {}
-            config.setdefault(component_name, default)
+            # everything else stores a single dict. Run the component's
+            # schema with {} so defaults get populated -- code paths like
+            # socket.FILTER_SOURCE_FILES expect a fully-populated mapping.
+            if component.multi_conf:
+                config.setdefault(component_name, [])
+            elif component_name not in config:
+                schema = component.config_schema
+                try:
+                    config[component_name] = schema({}) if schema is not None else {}
+                except Exception:  # noqa: BLE001
+                    # Schema requires explicit input we can't synthesize; fall
+                    # back to an empty mapping so subscripting at least returns
+                    # KeyError on missing keys rather than crashing on the
+                    # wrong type.
+                    config[component_name] = {}
 
     # Register platforms from the extra config (benchmark.yaml) so
     # USE_SENSOR, USE_LIGHT, etc. defines are emitted without needing

tests/components/json/__init__.py

@@ -0,0 +1,9 @@
+from tests.testing_helpers import ComponentManifestOverride
+
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    # json's to_code calls cg.add_library("bblanchon/ArduinoJson", ...). C++
+    # unit test builds that pull json in transitively (e.g. api) need that
+    # library registration to happen, otherwise json_util.cpp fails to find
+    # ArduinoJson.h.
+    manifest.enable_codegen()