support object-form template variable queries

[CoollZzz]

This pull request improves the robustness and compatibility of the metricFindQuery method in the DataSource class, ensuring it can handle both modern and legacy variable query formats, and adds comprehensive test coverage for these cases.

Enhancements to variable query handling:

• Added a new helper function variableQueryText to safely extract the query string from either a string or an object with a query field, ensuring type safety and flexibility.
• Updated the metricFindQuery method to use variableQueryText, rejecting any variable query that is neither a string nor an object with a string query field, and providing a clear error message in such cases.

Testing improvements:

• Added a new test suite for metricFindQuery covering:
  • Standard variable query objects with template expansion.
  • Legacy string-based variable queries for backward compatibility.
  • Error handling for invalid query object formats.