Builds the INSERT ... SELECT ... FROM unnest(...) query from Ecto building blocks.

Instead of assembling the whole SQL string by hand, we build an %Ecto.Query{}:

• the FROM source is the fragment unnest($1::t[], ...) AS u(col, ...). Column names and casts are dynamic (they depend on the column set), and fragment/1 as a macro requires a string literal — so from.source is built as an %Ecto.Query.FromExpr{} struct with properly alternating raw/expr parts (otherwise Ecto's Inspect blows up). The binding is named :s,
• the SELECT is assembled as a %Ecto.Query.SelectExpr{} struct (the same shape Ecto compiles from select: ^map): field(s, col) for unnest columns, type(^val, type) for placeholders, and fragment casts for JSON columns (f0."col"::jsonb) and custom-typed placeholders ($n::type) — those need a runtime cast string a dynamic/2 fragment/1 (literal-only) cannot express.

Execution and all of ON CONFLICT/RETURNING/prefix/struct loading are delegated to Ecto.Repo.insert_all/3. to_sql/1 renders the same plan into the full INSERT text with no database connection, via Ecto.Adapters.Postgres.Connection.