Trim direct ITensorMPS imports from tests, quarantine MPO cross-check

[mtfishman]

Summary

Pull the remaining direct ITensorMPS imports out of the test files that can trivially live without them, and quarantine the ones that genuinely need the ITensorMPS.MPO construction as a reference oracle.

Test changes:

• test/solvers/test_applyexp.jl, test/solvers/test_eigsolve.jl: switch using ITensorMPS: OpSum → using ITensors.Ops: OpSum. OpSum lives in ITensors.Ops; ITensorMPS just re-exports it, so there was no reason for these tests to depend on ITensorMPS for it.
• test/test_opsum_to_ttn.jl: trim to keep only the "Multiple onsite terms" regression test — the one testset that does not need ITensorMPS.MPO(os, sites) as a reference. Drop the using ITensorMPS: ITensorMPS line.
• test/test_opsum_to_ttn_mpo_cross_check.jl (new): houses the four testsets that compare ttn(H, is) against prod(ITensorMPS.MPO(H, sites)) — "OpSum to TTN", "OpSum to TTN QN", "OpSum to TTN Fermions", "OpSum to TTN QN missing". These are verbatim from the old file (plus their to_matrix helper). Quarantining them into a single file makes the remaining ITensorMPS test-time dependency easy to track and delete once there are internal equivalents for those correctness checks.

No src/ changes. ITensorMPS was never a dependency of the package itself (only of test/); this PR just reduces the test-time surface that pulls it in.

Version

Patch-bump to 0.18.1 (test-only diff, substantive per the standard VersionCheck classification).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.07%. Comparing base (dbb07fe) to head (153a6a0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #332      +/-   ##
==========================================
+ Coverage   71.04%   71.07%   +0.02%     
==========================================
  Files          70       70              
  Lines        3360     3360              
==========================================
+ Hits         2387     2388       +1     
+ Misses        973      972       -1     

Flag	Coverage Δ
docs	44.02% <ø> (+0.03%)	⬆️

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