[Feat] Add keywords merge weights and activate the top_k_files for FAST mode

[wangxingjun778]

Features

• Multi-file Evidence Aggregation: Transitioned the fast search mechanism from single-file to multi-file evidence aggregation for more comprehensive context retrieval.
• IDF-weighted Log-TF Scoring: Implemented a new ranking system using IDF-weighted log-TF scoring to improve result relevance.
• LLM Keyword Weighting: Updated the LLM prompt to provide keyword importance weights, enhancing the precision of the search logic.
• Dynamic Context & Fallbacks: Added support for dynamic context windows and introduced file-head fallbacks to ensure better evidence extraction when initial matches are insufficient.

Fixes

• IDF Scale Consistency: Corrected an inconsistency between the fallback IDF scale and the LLM-defined scale to ensure uniform scoring logic.
• Filename Search Pagination: Ensured that the filename search fallback correctly respects the top_k parameter, preventing unbounded result sets.

[gemini-code-assist]

Code Review

This pull request enhances the fast search mechanism by transitioning from single-file to multi-file evidence aggregation and implementing an IDF-weighted log-TF scoring system for ranking. The LLM prompt is updated to provide keyword importance weights, and the search logic now supports dynamic context windows and file-head fallbacks for better evidence extraction. Feedback focuses on correcting an inconsistency between the fallback IDF scale and the LLM-defined scale, as well as ensuring the filename search fallback respects the new top_k parameter.