address review: document single-component semantic, test first-match-wins

- Add docstring note explaining the JSON schema enforces exactly one
  component per config block (minProperties: 1, maxProperties: 1), and
  that "first match wins" is the defensive fallback if validation is
  bypassed
- Add test documenting that when multiple built-in fields are set,
  the first registry match wins

Assisted-by: Claude Opus 4.6

Author: MikeGoldsmith

opentelemetry-sdk/src/opentelemetry/sdk/_configuration/_common.py

@@ -92,6 +92,11 @@ def _resolve_component(
     field names on ``config``), then falls back to entry point loading
     for plugin components found in ``config.additional_properties``.
 
+    The JSON schema enforces exactly one component per config block
+    (``minProperties: 1, maxProperties: 1``). If multiple typed fields or
+    ``additional_properties`` entries are set (e.g. when schema validation
+    is bypassed), the first registry match wins.
+
     Args:
         config: A dataclass with ``additional_properties`` (from the
             ``@_additional_properties`` decorator).

opentelemetry-sdk/tests/_configuration/test_common.py

@@ -361,3 +361,13 @@ def test_plugin_not_found_raises_configuration_error(self):
                 _resolve_component(
                     config, self.registry, "test_group", "test component"
                 )
+
+    def test_first_registry_match_wins_when_multiple_set(self):
+        """When multiple built-in fields are set (which the schema should
+        prevent), the first registry match wins."""
+        config = self.cls(builtin_a={"a": 1}, builtin_b="b")
+        result = _resolve_component(
+            config, self.registry, "test_group", "test component"
+        )
+        # builtin_a comes first in the registry dict
+        self.assertEqual(result, ("resolved_a", {"a": 1}))