feat: add swanlab.merge_callbacks API and callbacker tests

[SAKURA-CAT]

Summary

• 新增 swanlab.merge_callbacks() 公开 API，允许用户在 swanlab.init() 之前注册全局回调
• 重构 CallbackManager，移除 callbacks_dict 构造参数，统一通过 merge_callbacks 注入
• 新增 callbacker 模块单元测试（12 cases）

Changes

文件	说明
swanlab/sdk/cmd/merge_callbacks.py	新增 merge_callbacks 命令实现
swanlab/__init__.py	对外暴露 merge_callbacks
swanlab/__init__.pyi	添加 merge_callbacks 类型声明
swanlab/sdk/__init__.py	导出 merge_callbacks
swanlab/sdk/internal/context/components/callbacker/__init__.py	重构 CallbackManager
tests/unit/sdk/internal/context/components/callbacker/test_callbacker.py	新增单元测试

Usage

import swanlab
from swanlab import Callback

class MyCallback(Callback):
    @property
    def name(self) -> str:
        return "my_callback"

    def on_run_initialized(self, run_dir, path):
        print("Run initialized!")

# 单个
swanlab.merge_callbacks(MyCallback())
# 多个
swanlab.merge_callbacks([MyCallback(), AnotherCallback()])

swanlab.init()

[gemini-code-assist]

Code Review

This pull request introduces the merge_callbacks function to the SwanLab SDK, allowing users to register custom callbacks before run initialization. The internal architecture has been refactored to support both global and local callback registries, ensuring that local callbacks take precedence. The review feedback focuses on improving type hint consistency across the SDK by suggesting the use of Union[Iterable[Callback], Callback] for callback parameters in several internal functions and ensuring the necessary Union typing import is included. Additionally, a refactoring opportunity was identified to simplify callback merging if internal methods are updated to handle the union type directly.