Move test-only helpers (random_tensornetwork, random_ttn, random_mps, ModelHamiltonians) out of src/

[mtfishman]

Summary

Pull non-basic constructors that are only used by tests out of src/ and into a new test/utils.jl. Each test that needs them does include("utils.jl") inside its gensym module. The file is intentionally named utils.jl so the ITensorPkgSkeleton.runtests runner does not auto-pick it up (the runner only globs test_*.jl).

What moved to test/utils.jl:

• random_tensornetwork (12 method specializations, including the Distribution- and IsUnderlyingGraph-based variants) — moved from src/specialitensornetworks.jl.
• random_ttn and random_mps — moved from src/treetensornetworks/treetensornetwork.jl. These thin wrappers around random_tensornetwork had no real src/ consumers (only docstring examples) and a single test reference.
• ModelHamiltonians.tight_binding, heisenberg, ising plus the internal helpers to_callable, nth_nearest_neighbors, next_nearest_neighbors — moved from src/lib/ModelHamiltonians/ into a ModelHamiltonians submodule of test/utils.jl. Test sites continue to call them as ModelHamiltonians.ising(...) etc.

What was deleted entirely (no consumers):

• delta_network (and the rest of src/specialitensornetworks.jl) — only consumer was ModelNetworks.ising_network, which itself had no consumers.
• ModelHamiltonians.hubbard — no consumers.
• src/lib/ModelNetworks/ (ising_network, ising_network_state) — no consumers; the lone using ITensorNetworks.ModelNetworks: ModelNetworks in test/test_belief_propagation.jl was an unused import and was dropped.
• random_ttn / random_mps jldoctest examples in src/normalize.jl, src/expect.jl, src/treetensornetworks/abstracttreetensornetwork.jl, plus [ref] links in See also: lists. Consistent with not having doc examples that depend on functions being removed.

Construction pattern

The random_tensornetwork helpers in test/utils.jl are factored through a private _random_tensornetwork(s::IndsNetwork, build_tensor; link_space) that uses the explicit Dict-of-ITensor pattern: build a Dict{Edge, Index} of fresh link indices over edges(s) (mirrored under reverse(e)); for each vertex, gather the site indices from vertex_data(s)[v] (or empty if v has no entry) plus the link indices on incident edges; let the caller-supplied build_tensor produce the per-vertex ITensor; wrap with the basic ITensorNetwork(::Dict{V, ITensor}) constructor. The 12 user-facing random_tensornetwork method specializations and their IsUnderlyingGraph trait wrappers route through this helper.

Documentation

• docs/src/{computing_properties,solvers}.md setup blocks now build a small inline random_state(g, s; link_space) helper using the same Dict-of-ITensor pattern, instead of calling random_ttn. link_space is required (no default), passed explicitly at the call sites.
• docs/src/tree_tensor_networks.md drops the random_ttn / random_mps constructor examples and autodoc entries.
• docs/src/reference.md's @autodocs listing trimmed to just ITensorNetworks (no longer pulling in the removed ModelNetworks / ModelHamiltonians submodules).

Incidental import cleanups

A few imports were previously available inside ITensorNetworks as a side effect of the deleted files' using clauses; now imported directly:

• ITensors: delta — src/formnetworks/bilinearformnetwork.jl, src/caches/abstractbeliefpropagationcache.jl

Version + test deps

• Bump to 0.19.0 (pre-1.0 minor bump = breaking, public API removed).
• Update pinned ITensorNetworks compat in test/, docs/, examples/ Project.toml.
• Add SimpleTraits to test/Project.toml so test/utils.jl can use @traitfn / IsUnderlyingGraph.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.50%. Comparing base (31bb354) to head (6b1ebef).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #339      +/-   ##
==========================================
+ Coverage   71.07%   72.50%   +1.43%     
==========================================
  Files          70       67       -3     
  Lines        3360     3179     -181     
==========================================
- Hits         2388     2305      -83     
+ Misses        972      874      -98     

Flag	Coverage Δ
docs	46.24% <ø> (+2.22%)	⬆️

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