Add _state == McpClientState.STARTING guard inside the threading.RLock in
start() and _ensure_started() so that concurrent callers see the in-progress
state and return immediately, preventing double initialisation of the MCP
server connection.

- Move _CountingTransport test double from inline step file to dedicated
  features/mocks/counting_mcp_transport.py per CONTRIBUTING.md mock placement rules
- Updated step definitions to import CountingMCPTransport from new location
- Added Race condition entry to CHANGELOG under [Unreleased] -> Fixed
- Added contributor credit for McpClient race condition fix

ISSUES CLOSED: #10438

@@ -0,0 +100,4 @@
+
+    def _render_node(node: object, prefix: str, is_last: bool) -> None:
+        connector = "L-- " if is_last else "+-- "
+        lines.append(f"{prefix}{connector}{node.label}")  # type: ignore[union-attr]

@@ -0,0 +102,4 @@
+        connector = "L-- " if is_last else "+-- "
+        lines.append(f"{prefix}{connector}{node.label}")  # type: ignore[union-attr]
+        child_prefix = prefix + ("    " if is_last else "|   ")
+        for i, child in enumerate(node.children):  # type: ignore[union-attr]

@@ -0,0 +103,4 @@
+        lines.append(f"{prefix}{connector}{node.label}")  # type: ignore[union-attr]
+        child_prefix = prefix + ("    " if is_last else "|   ")
+        for i, child in enumerate(node.children):  # type: ignore[union-attr]
+            _render_node(child, child_prefix, i == len(node.children) - 1)  # type: ignore[union-attr]

@@ -0,0 +107,4 @@
+
+    root = element.root
+    lines.append(root.label)
+    for i, child in enumerate(root.children):  # type: ignore[union-attr]