fix: remove trailing comma that wrapped triples result in tuple in _invoke

[octo-patch]

Fixes #743

Problem

In the synchronous _invoke method of SchemaFreeExtractor, the result of triples_extraction was accidentally wrapped in a 1-element tuple due to a trailing comma:

# Before (buggy)
triples = (self.triples_extraction(passage, filtered_entities),)

When assemble_sub_graph_with_triples iterates for tri in triples:, it received the entire list of triples as a single element rather than iterating individual triples. The check if tri is None or len(tri) != 3: then always skipped this element (since the list length is not 3), causing all relationships to be silently dropped in synchronous extraction mode.

The async _ainvoke path was unaffected because it uses asyncio.gather, which unpacks results correctly into triples directly.

Solution

Remove the trailing comma so triples holds the list returned by triples_extraction directly:

# After (fixed)
triples = self.triples_extraction(passage, filtered_entities)

Testing

The fix aligns the synchronous _invoke behavior with the async _ainvoke path, which already worked correctly. The one-character change (removing the trailing comma) is the minimal fix needed.