Add unit tests for validation logic in QueryGenerator::generateQuery()

[sahitya-chandra]

fixes #91 (follow up pr after #89)

Summary

Adds validation for natural language query input and unit tests to cover the validation logic used by QueryGenerator::generateQuery().

Changes

• Validation in utils: Introduced validate_natural_language_query() in utils to enforce:
  • Non-empty input (rejects empty and whitespace-only strings)
  • Maximum length (configurable via max_query_length)
• QueryGenerator integration: QueryGenerator::generateQuery() now calls this validator before proceeding and returns a QueryResult with success = false and a clear error message when validation fails.
• Unit tests: New tests in tests/unit/test_utils.cpp for:
  • Empty query → "Query cannot be empty."
  • Whitespace-only query → "Query cannot be empty."
  • Query exceeding max length → error message with limit and actual length
  • Valid short query → no error
  • Query exactly at max length → no error

Testing

• New unit tests added; run make test-setup && ./build_tests/tests/pg_ai_query_tests --gtest_filter="UtilsTest.ValidateNaturalLanguageQuery_*"

Note: Google Test filter = UtilsTest.ValidateNaturalLanguageQuery_*
[==========] Running 5 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 5 tests from UtilsTest
[ RUN      ] UtilsTest.ValidateNaturalLanguageQuery_Empty
[       OK ] UtilsTest.ValidateNaturalLanguageQuery_Empty (0 ms)
[ RUN      ] UtilsTest.ValidateNaturalLanguageQuery_WhitespaceOnly
[       OK ] UtilsTest.ValidateNaturalLanguageQuery_WhitespaceOnly (0 ms)
[ RUN      ] UtilsTest.ValidateNaturalLanguageQuery_TooLong
[       OK ] UtilsTest.ValidateNaturalLanguageQuery_TooLong (0 ms)
[ RUN      ] UtilsTest.ValidateNaturalLanguageQuery_ValidShort
[       OK ] UtilsTest.ValidateNaturalLanguageQuery_ValidShort (0 ms)
[ RUN      ] UtilsTest.ValidateNaturalLanguageQuery_ExactlyMaxLength
[       OK ] UtilsTest.ValidateNaturalLanguageQuery_ExactlyMaxLength (0 ms)
[----------] 5 tests from UtilsTest (1 ms total)

[----------] Global test environment tear-down
[==========] 5 tests from 1 test suite ran. (1 ms total)
[  PASSED  ] 5 tests.