fix(falkordb): short-circuit fulltext query when every token was a stopword

[aasc77]

Summary

_build_falkor_fulltext_query (and the equivalent class-method FalkorDriver.build_fulltext_query) emit (@group_id:"X") () whenever the input's content tokens reduce to all stopwords after sanitize + filter — FalkorDB/RediSearch rejects the trailing empty group with Syntax error at offset N near X. There's a too-long guard above the wrap site but no symmetric too-short/empty one. This PR adds it.

sanitized_query = ' | '.join(filtered_words)

# new: short-circuit when every input token was a stopword
if not filtered_words:
    return ''

# existing: too-long guard
if len(sanitized_query.split(' ')) + len(group_ids or '') >= max_query_length:
    return ''

Every call site already special-cases the '' return as "no candidates" (if fuzzy_query == '': return [] in node_fulltext_search / edge_fulltext_search / the community siblings), so no caller-side change is required — the new guard piggybacks on the existing sentinel.

How I hit it

During a 230-episode bulk replay against a 1849-entity FalkorDB graph, the dedup queue stalled at 31% when an episode's LLM-derived content tokens happened to be all stopwords. Container log (graphiti-core 0.28.2 + FalkorDB v4.18.10):

graphiti_core.driver.falkordb_driver - ERROR - Error executing FalkorDB query: Query timed out
{'query': '(@group_id:"X") ()', 'limit': 20, 'routing_': 'r', 'edge_uuids': [], 'group_ids': ['X']}
services.queue_service - ERROR - Failed to process episode None for group X: RediSearch: Syntax error at offset 26 near X

Reproducing the parse error directly against FalkorDB:

$ docker exec falkordb redis-cli GRAPH.QUERY mygraph \
    'CALL db.idx.fulltext.queryRelationships("RELATES_TO", "(@group_id:\"X\") ()") YIELD relationship RETURN relationship LIMIT 1'
RediSearch: Syntax error at offset 26 near X

With this PR applied the replay completed cleanly.

Test plan

• Existing call sites already handle '' as "no candidates" — verified by grep across graphiti_core/driver/falkordb/operations/search_ops.py (4 sites: node_fulltext_search, edge_fulltext_search, community node + edge siblings).
• Sanity: non-stopword input still produces a wrapped query of the expected shape (regression-tested downstream in our consumer's QA suite).
• No callers depend on a malformed-empty return — the broken-() output was never a valid RediSearch expression so any caller that did something with it was broken anyway.

Happy to add a unit test directly inside this repo if a maintainer points me at the right place — I can see the testing structure under tests/ and would just want to confirm the location/style conventions.

Notes

• Two-file patch because the same builder lives in both graphiti_core/driver/falkordb_driver.py (class-method form on FalkorDriver.build_fulltext_query) and graphiti_core/driver/falkordb/operations/search_ops.py (module-level _build_falkor_fulltext_query). Same guard, same comment, applied to both.
• Comment in the patch documents why the early return is safe (callers already special-case '').

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[aasc77]

I have read the CLA Document and I hereby sign the CLA behalf on myself, e-mail: angel.s@me.com