Add Modular Testing Suite and Test Harness for HealthLLM.jl

[harshitchand998-ai]

This PR implements Issue #32 by introducing a modular testing suite and test harness for HealthLLM.jl.

What changed

• Added a modular test harness with reusable executors and validators.
• Added exact, normalized text, content, structured, and execution-based validation.
• Added mock LLM and RAG testing support for deterministic offline tests.
• Added golden reference fixtures and synthetic test corpus.
• Added unit tests for utilities, schemas, database functionality, and the harness itself.
• Added query-generation, LLM, golden regression, FunSQL execution, and end-to-end RAG tests.
• Updated test/runtests.jl to integrate the new test suites.
• Added documentation describing the testing harness and testing workflow.
• Extended query functionality required by the new test coverage.

Testing
The new test suite is designed to run primarily without external LLM APIs or PostgreSQL dependencies, while allowing live integrations to remain opt-in.

Closes #32

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.78%. Comparing base (677f98e) to head (982aeb5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #54   +/-   ##
=======================================
  Coverage   60.78%   60.78%           
=======================================
  Files           4        4           
  Lines         102      102           
=======================================
  Hits           62       62           
  Misses         40       40           

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[harshitchand998-ai]

@ParamThakkar123 can you pls review it.