CompatHelper: bump compat for ITensorMPS to 0.4 for package test, (keep existing compat)

[ITensorBot]

Summary

Bumps compat for ITensorMPS to 0.4 in test/Project.toml. ITensorMPS is a test-only dependency in this repo.

Also bumps the package version 0.1.7 → 0.1.8 to satisfy VersionCheck (subdir Project.toml changes are classified as substantive).

Rebased onto current main.

Note: CheckCompatBounds blocks on ITensorVisualizationBase 0.1.18 being registered first (root depends on ITensorVisualizationBase, which only adds ITensorMPS 0.4 compat in 0.1.18). Once ITensorVisualizationBase 0.1.18 lands and registers, this PR's CI should pass.

Test plan

• CI green (gated on ITensorVisualizationBase 0.1.18 registration)

[mtfishman]

Closing+reopening to retrigger CI now that ITensorVisualizationBase 0.1.18 is registered.