fix: prevent null boundaries in migration when array is empty

[Copilot]

The migration 1769099632447_boundary_config_add_id.ts uses jsonb_agg which returns null when aggregating zero rows, causing empty boundaries arrays to become null.

Changes:

• Wrapped jsonb_agg calls in both up and down migrations with COALESCE(jsonb_agg(...), '[]'::jsonb) to preserve empty arrays

-- Before
SELECT jsonb_agg(
  jsonb_set(boundary, '{id}', to_jsonb(gen_random_uuid()::text))
)

-- After  
SELECT COALESCE(
  jsonb_agg(
    jsonb_set(boundary, '{id}', to_jsonb(gen_random_uuid()::text))
  ),
  '[]'::jsonb
)

This ensures inspector configs with empty boundaries arrays maintain their structure through migrations.

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