refactor(client): validate direction/max_depth before proto import in traverse()

Pure string/int validation now fires before the deferred proto import.
Callers get ValueError for invalid inputs even when proto stubs have not
been generated, keeping validation independent of the build step.

Addresses review thread #28.

Author: polaz

coordinode/coordinode/client.py

@@ -383,6 +383,15 @@ async def traverse(
         Returns:
             :class:`TraverseResult` with ``nodes`` and ``edges`` lists.
         """
+        # Validate pure string/int inputs before importing proto stubs — ensures ValueError
+        # is raised even when proto stubs have not been generated yet.
+        _valid_directions = {"outbound", "inbound", "both"}
+        key = direction.lower()
+        if key not in _valid_directions:
+            raise ValueError(f"Invalid direction {direction!r}. Must be one of: 'outbound', 'inbound', 'both'.")
+        if max_depth < 1:
+            raise ValueError(f"max_depth must be >= 1, got {max_depth!r}.")
+
         from coordinode._proto.coordinode.v1.graph.graph_pb2 import (  # type: ignore[import]
             TraversalDirection,
             TraverseRequest,
@@ -393,11 +402,6 @@ async def traverse(
             "inbound": TraversalDirection.TRAVERSAL_DIRECTION_INBOUND,
             "both": TraversalDirection.TRAVERSAL_DIRECTION_BOTH,
         }
-        key = direction.lower()
-        if key not in _direction_map:
-            raise ValueError(f"Invalid direction {direction!r}. Must be one of: 'outbound', 'inbound', 'both'.")
-        if max_depth < 1:
-            raise ValueError(f"max_depth must be >= 1, got {max_depth!r}.")
         direction_value = _direction_map[key]
 
         req = TraverseRequest(