Fix mermaid diagram subgraph generation

[LeonardoNatale]

Issue

• Current mermaid diagram are not correctly generated, resulting in empty subgraph.
• See the jaffle_shop example ⬇️

Solution

• Remove extra sort on the schema_nodes -> it looks like its exhausting the iterator (not 100% sure)
• handle case where node length > 2
  • the inner for loop will fail for a node represented by the tuple (analytics, finance, kpis)
• see diagram generated following the changes ⬇️

[MaxHalford]

I just added some doctests for good measure :)