backend: Refactor Retriever/Function tools into single Tool concept

[tianjing-li]

• Removes the notion of Retriever or Function tools, they are now all considered tools
• Now all tools should inherit from BaseTool
• Updates all existing config and imports
• Updates unit tests

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.14286% with 1 lines in your changes are missing coverage. Please review.

Please upload report for BASE (main@40bd862). Learn more about missing BASE report.

Files	Patch %	Lines
src/backend/tools/tavily.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #154   +/-   ##
=======================================
  Coverage        ?   88.62%           
=======================================
  Files           ?      113           
  Lines           ?     3622           
  Branches        ?        0           
=======================================
  Hits            ?     3210           
  Misses          ?      412           
  Partials        ?        0           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[lusmoura]

🚀 just one nit