chore: clean up deprecated search code after PaperSearchService migration

[jerry609]

问题

PaperSearchService 已上线，以下旧代码可以清理：

• utils/search.py（SemanticScholarSearch）— 仅 ContextEngine legacy fallback 还在引用
• topic_search_sources.py 的 PapersCoolTopicSource/ArxivTopicSource/HFDailyTopicSource
• paperscool_topic_search.py — paperscool.py 已将其设为 None，仅作 test hook
• ContextEngine 的 paper_searcher 参数

前置

• refactor: remove legacy SemanticScholarSearch fallback from ContextEngine #78 ContextEngine legacy path 清理
• refactor: wire HarvestPipeline search through PaperSearchService #79 HarvestPipeline 切换

方案

• 删除废弃文件/类
• 更新所有 import 引用
• 确保 pytest 通过

[jerry609]

Completed in commit 55f4b1e. Deleted PapersCoolTopicSearchWorkflow, TopicSearchSourceRegistry, SemanticScholarSearch and updated all test mocks.