refactor(protobuf): single-pass enum emit, use Datatypes.STRING[0]

- Collapse two-pass loop into one: _write_nested_enum is called
  immediately before the field declaration, removing the redundant
  pre-iteration.
- Replace "string" literal with Datatypes.STRING[0] from datatypes module.
- Update expected.proto: enum definitions now appear inline before their
  respective fields rather than grouped at the top of the message.

Co-Authored-By: Claude and aki1770-del <aki1770@gmail.com>

Author: aki1770-del

src/vss_tools/exporters/protobuf.py

@@ -25,6 +25,7 @@
     VSSDataDatatype,
     VSSDataStruct,
 )
+from vss_tools.datatypes import Datatypes
 from vss_tools.tree import VSSNode
 
 PATH_DELIMITER = "."
@@ -183,21 +184,13 @@ def _write_nested_enum(fd: TextIOWrapper, field_name: str, allowed: list, indent
 def print_messages(
     nodes: tuple[VSSNode], fd: TextIOWrapper, static_uid: bool, add_optional: bool, include_comments: bool
 ):
-    # Pass 1: write nested enum definitions for every string field with allowed values.
-    for node in nodes:
-        if isinstance(node.data, VSSDataDatatype):
-            base = node.data.datatype.strip("[]")
-            if base == "string" and node.data.allowed:
-                _write_nested_enum(fd, node.name, node.data.allowed)
-
-    # Pass 2: write field declarations.
     usedKeys: dict[int, str] = {}
     for i, node in enumerate(nodes, 1):
         if isinstance(node.data, VSSDataDatatype):
             dt_val = node.data.datatype
             base = dt_val.strip("[]")
-            if base == "string" and node.data.allowed:
-                # Replace plain string with the nested enum type.
+            if base == Datatypes.STRING[0] and node.data.allowed:
+                _write_nested_enum(fd, node.name, node.data.allowed)
                 data_type = _enum_type_name(node.name)
             else:
                 data_type = mapped.get(base, base)

tests/vspec/test_protobuf_enum_allowed/expected.proto

@@ -8,13 +8,13 @@ message A {
     FM = 2;
     BLUETOOTH = 3;
   }
+  SourceEnum Source = 1;
   enum SurfaceEnum {
     DRY = 0;
     WET = 1;
     SNOW = 2;
     ICE = 3;
   }
-  SourceEnum Source = 1;
   repeated SurfaceEnum Surface = 2;
   float Speed = 3;
 }