Contract networks mixing operators and plain tensors

[mtfishman]

Summary

contract_network now handles a network that mixes operator-valued (NamedTensorOperator) and plain tensors, which previously threw a convert MethodError. It promotes the operands to their common type with promote_type and convert before lowering to the lazy expression, so every operand shares one concrete type and the contraction folds with *, routing operators through operator_product and preserving the output and input pairing. Without that, a mixed network widens the symbolic Mul container to a UnionAll that cannot be constructed. An all-plain network is unaffected (the promotion is a no-op).

The promotion lives in ITensorBase: eager operands via ITensor/ITensorBase.jl#235 and lazy operands via ITensor/ITensorBase.jl#236. With the lazy case handled, this drops the interim state-unwrap workarounds in belief propagation: vertex_scalar and updated_message contract operator messages directly, and the NormNetwork message_update! re-partitions the result with operator (the vertex factor is a lazy ket * conj(bra) product, so its surviving bond legs come out unpaired and the message's ket/bra pairing is assigned by the norm-network convention).

Also switches the NormNetwork tests to the package's contract_network rather than a local helper of the same name that shadowed it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.81%. Comparing base (4b43b57) to head (257e09a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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+ Coverage   84.79%   84.81%   +0.02%     
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  Files          15       15              
  Lines         684      685       +1     
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+ Hits          580      581       +1     
  Misses        104      104              

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