refactor: improve search execution and importer strategies

[harsh20k]

Summary

• extract search plan entries/executor to reduce duplication and clarify permissions (addresses duplication/complex method smell)
• add strategy pattern for Jira auth tokens and project/board imports (replaces conditionals with polymorphism)
• factor shared search serializer fields into a base class; update refactoring_log.md (pull-up fields, push-down specializations)

Smells Addressed

• Duplication in search orchestration (search* helpers) reduced via SearchPlanEntry + SearchTaskExecutor
• Conditional logic in importer token handling and project/board selection replaced with strategies
• Long/duplicated serializer field definitions consolidated into BaseSearchResultsSerializer
• Documented all refactorings in refactoring_log.md