Fix SPARQL syntax errors after query alterations

Problem/Motivation

When modules alter SPARQL queries using hook_query_alter() or hook_query_TAG_alter(), they may introduce invalid SPARQL syntax, particularly in SELECT clauses. This commonly occurs when multiple variables are selected with incorrect comma placement after DISTINCT.

Steps to reproduce

1. Implement a hook_query_alter() that modifies the SELECT clause to include multiple variables
2. Execute the SPARQL query
3. Observe parse error: Encountered " "," ", "" at line 1, column 20

Proposed resolution

Add a fixSparqlSyntax() method in the Query::compile() method to sanitize SPARQL syntax after hook alterations.

Remaining tasks

User interface changes

API changes

Data model changes