refactor: extract DConfig loading and add index-level mutex for thread safety

[liyigang1]

Extracted all DConfig loading logic from searchutility.cpp into a dedicated SearchDConfig class with a SearchDConfigSnapshot struct. DConfig reads via DBus are not thread-safe, so all reads are now performed on the main thread and the resulting snapshot is passed to worker threads via custom options.

Added IndexMutexGuard to provide per-index-directory mutual exclusion, preventing concurrent access to the same Lucene index directory since FSDirectory::open() returns cached instances sharing underlying buffers.

These changes enable:

1. Thread-safe DConfig access by restricting reads to the main thread
2. Safe concurrent searching across multiple index directories
3. Cleaner separation of configuration loading concerns

Influence:

1. Verify multi-threaded search scenarios no longer cause index corruption
2. Ensure DConfig snapshot is correctly propagated to all search strategies
3. Test path prefix query optimization with dynamic indexed directory changes

refactor: 提取 DConfig 加载逻辑并添加索引级互斥锁以保障线程安全
Bug: https://pms.uniontech.com/bug-view-361809.html

将 searchutility.cpp 中所有 DConfig 加载逻辑提取到独立的 SearchDConfig 类， 使用 SearchDConfigSnapshot 结构体承载配置快照。DConfig 通过 DBus 读取， 非线程安全，因此所有读取操作限定在主线程执行，生成的快照通过自定义选项
传递给工作线程。

新增 IndexMutexGuard 提供按索引目录的互斥机制，防止多线程并发访问同一
Lucene 索引目录，因为 FSDirectory::open() 对相同路径返回缓存实例，
底层 IndexInput 缓冲区共享。

这些改动实现了：

1. 通过将 DConfig 读取限制在主线程来保障线程安全
2. 支持多索引目录间的安全并发搜索
3. 配置加载职责的清晰分离

Influence:

1. 验证多线程搜索场景不再导致索引损坏
2. 确保 DConfig 快照正确传递到所有搜索策略
3. 测试索引目录动态变化时路径前缀查询优化的正确性

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