core: fix ambiguous column error when searching or sorting

[nurikk]

fix ambiguous column error when searching or sorting by multiple fields from same relationship

Fixes SQLite 'ambiguous column name' error that occurred when users configured column_searchable_list or column_sortable_list with multiple fields from the same related model (e.g., ['user.id', 'user.name']).

The issue happened because:

• Each field was independently joining the same table multiple times
• sort_query runs before search_query, so duplicate joins could occur across methods
• SQLite cannot resolve ambiguous column references with duplicate table joins

Solution:

• Added _get_joined_entities() to inspect existing joins in the SQL statement
• Added _join_relationship_paths() as DRY helper to track and prevent duplicate joins
• Refactored both search_query and sort_query to use the helper
• Joins are now tracked both by relationship path (within method) and by inspecting the statement (cross-method), ensuring no duplicates

Users can now safely use multiple fields from the same relationship in searchable and sortable columns without encountering database errors.