refactor(provider): use LiteLLM as unified LLM requester backend

- Replace 23+ individual requester implementations with unified litellmchat.py
  - Add litellm_provider field to 27 YAML manifests for provider routing
  - Delete redundant requester subclasses
  - Add unit tests for LiteLLMRequester (29 tests)
  - Fix num_retries parameter name (was max_retries)
  - Fix exception handling order for subclass exceptions

  LiteLLM provides unified API for 100+ providers, eliminating need for
  provider-specific requesters.

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

@@ -1,67 +0,0 @@
-from __future__ import annotations
-
-import typing
-
-from . import chatcmpl
-from .. import errors, requester
-import langbot_plugin.api.entities.builtin.resource.tool as resource_tool
-import langbot_plugin.api.entities.builtin.pipeline.query as pipeline_query
-import langbot_plugin.api.entities.builtin.provider.message as provider_message
-
-
-class DeepseekChatCompletions(chatcmpl.OpenAIChatCompletions):
-    """Deepseek ChatCompletion API 请求器"""
-
-    default_config: dict[str, typing.Any] = {
-        'base_url': 'https://api.deepseek.com',
-        'timeout': 120,
-    }
-
-    async def _closure(
-        self,
-        query: pipeline_query.Query,
-        req_messages: list[dict],
-        use_model: requester.RuntimeLLMModel,
-        use_funcs: list[resource_tool.LLMTool] = None,
-        extra_args: dict[str, typing.Any] = {},
-        remove_think: bool = False,
-    ) -> tuple[provider_message.Message, dict]:
-        self.client.api_key = use_model.provider.token_mgr.get_token()
-
-        args = {}
-        args['model'] = use_model.model_entity.name
-
-        if use_funcs:
-            tools = await self.ap.tool_mgr.generate_tools_for_openai(use_funcs)
-
-            if tools:
-                args['tools'] = tools
-
-        # 设置此次请求中的messages
-        messages = req_messages
-
-        # deepseek 不支持多模态，把content都转换成纯文字
-        for m in messages:
-            if 'content' in m and isinstance(m['content'], list):
-                m['content'] = ' '.join([c['text'] for c in m['content'] if 'text' in c])
-
-        args['messages'] = messages
-
-        # 发送请求
-        resp = await self._req(args, extra_body=extra_args)
-
-        # print(resp)
-
-        if resp is None:
-            raise errors.RequesterError('接口返回为空，请确定模型提供商服务是否正常')
-        # 处理请求结果
-        message = await self._make_msg(resp, remove_think)
-
-        # Extract token usage from response
-        usage_info = {}
-        if hasattr(resp, 'usage') and resp.usage:
-            usage_info['input_tokens'] = resp.usage.prompt_tokens or 0
-            usage_info['output_tokens'] = resp.usage.completion_tokens or 0
-            usage_info['total_tokens'] = resp.usage.total_tokens or 0
-
-        return message, usage_info