restore: restore deleted provider requester files

Restore individual provider requester implementations that were
removed in de61b5d3. These files coexist with the unified
litellmchat.py backend.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/langbot/pkg/provider/modelmgr/requesters/volcarkchatcmpl.py

@@ -0,0 +1,17 @@
+from __future__ import annotations
+
+import typing
+import openai
+
+from . import chatcmpl
+
+
+class VolcArkChatCompletions(chatcmpl.OpenAIChatCompletions):
+    """火山方舟大模型平台 ChatCompletion API 请求器"""
+
+    client: openai.AsyncClient
+
+    default_config: dict[str, typing.Any] = {
+        'base_url': 'https://ark.cn-beijing.volces.com/api/v3',
+        'timeout': 120,
+    }