[api] Derive decode cases from the type's wire type

decode_case() reads wire_type instead of taking it at every call, the
expression and the store statement are two small hooks that repeated
fields override, and message fields build one body. No generated case
declares a local any more, so the braced form and its test go; the
compiler rejects a jump over a local if one ever appears. The wire
type test now proves a dropped frame with an ordering marker instead
of assuming it, and shares StateWaiter.

tests/unit_tests/components/api/test_api_protobuf_generator.py

@@ -188,11 +188,10 @@ def test_multi_byte_tag_fixed32_falls_back_to_the_generic_helper(
 
 def _decode_case(field_type: int, number: int) -> str:
     """Return the decode_field() case the generator emits for one decoded field."""
-    field = descriptor_pb2.FieldDescriptorProto(
-        name="value", number=number, type=field_type
-    )
-    ti = create_field_type_info(field, needs_decode=True, needs_encode=False)
-    return ti.decode_content
+    field = _field(field_type, number)
+    return create_field_type_info(
+        field, needs_decode=True, needs_encode=False
+    ).decode_content
 
 
 @pytest.mark.parametrize(
@@ -236,12 +235,3 @@ def test_message_gets_a_single_decode_field_override() -> None:
     assert "const ProtoFieldValue value(data, scalar);" in cpp
     for number, wire_type in ((1, 2), (2, 0), (3, 5)):
         assert f"case PROTO_DECODE_CASE({number}, {wire_type}):" in cpp, cpp
-
-
-def test_multi_statement_decode_cases_are_scoped() -> None:
-    """Bodies with several statements or locals get their own block so no jump crosses an initialization."""
-    case = _decode_case(BYTES, 4)
-    lines = case.splitlines()
-    assert lines[0] == "case PROTO_DECODE_CASE(4, 2): {", case
-    assert lines[-1] == "}", case
-    assert "value.data();" in case and "value.size();" in case