Comprehensive SPARQL tests

[daghovland]

Added tests for several examples in the SPARQL-1.2 specification

This pull request implements support for SPARQL aggregate functions and the GROUP BY clause, enabling complex analytical queries over RDF datasets.

Key Changes

• AST & Parser Enhancements:

  • Extended the query AST in src/Rdf/Query.fs with Aggregate, Expression, and ProjectionElement types.
  • Updated Sparql.Parser to handle aggregate expressions (e.g., (SUM(?price) AS ?total)) and GROUP BY conditions.
  • Renamed internal union cases (e.g., Variable to ExprVariable) for better cross-language compatibility (C#/F#).

• Query Processor Implementation (src/Rdf/QueryProcessor.fs):

  • Implemented evaluation logic for GROUP BY clauses.
  • Added support for both explicit and implicit grouping (where aggregates are used without a GROUP BY clause).
  • Implemented the following SPARQL aggregate functions:
    • SUM (with DISTINCT support).
    • COUNT (including COUNT(*) and COUNT(?var) with DISTINCT support).
    • MIN and MAX.
    • AVG (with DISTINCT support).
  • Ensured spec compliance, such as returning 0 for SUM on empty groups.

• Testing & Verification:

  • Enabled and passed the TestSparql12ExampleAggregate parser test.
  • Implemented a new integration test TestSparqlAggregate in Api.Tests/TestApi.cs to verify end-to-end execution of aggregate queries via the public IDataset API.
  • Verified all existing SPARQL tests to ensure no regressions.

Example Supported Query

PREFIX : <http://books.example/>
SELECT ?org (SUM(?lprice) AS ?totalPrice)
WHERE {
  ?org :hasBook ?book .
  ?book :price ?lprice .
}
GROUP BY ?org