fix: correct NEO_DB singleton mock in gap_analysis test

[Subh24ai]

Problem

test_tiered_execution_optimization was failing with:
TypeError: NEO_DB.gap_analysis() missing 1 required positional argument: 'name_2'

This happened because the test called db.NEO_DB.gap_analysis() directly
on the class, but NEO_DB is a singleton that must be accessed via
NEO_DB.instance().

Fix

Updated setUp() to instantiate NEO_DB correctly using NEO_DB.instance()
and call gap_analysis() on the instance instead of the class.

Result

Test now passes without requiring a running Neo4j instance.