refactor(answer): improve debug log messages for clarity and readability

[wsxiaoys]

• Updated debug log message in answer.rs to clarify the content being logged and the number of matched documents.
• Improved debug log message in merge_code_snippets to make it more descriptive and easier to understand.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 63.17%. Comparing base (b8e2c4b) to head (0d0bc74).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
ee/tabby-webserver/src/service/answer.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3669      +/-   ##
==========================================
- Coverage   63.18%   63.17%   -0.01%     
==========================================
  Files         207      207              
  Lines       24862    24862              
==========================================
- Hits        15709    15707       -2     
- Misses       9153     9155       +2     

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