Fix variable name-collision guard to match updated JuMP error message

[magnuask]

Summary

Fixes #96 (#96).

The test suite fails during "General tests" and "Variable creation" with:

An object of name :sink_surplus is already registered in this model.

Root cause

When building variables for a type hierarchy, variables_elements (and variables_element_ext_data) in src/model.jl intentionally call variables_element for a supertype and then again for its subtypes. To avoid double-registration of the same JuMP variable name, the call is wrapped in a try/catch that silently continues when the raised ErrorException corresponds to an already-registered name.

The guard matched the error text against the hardcoded substrings:

• "An object of name"
• "is already attached to this model."

Newer JuMP (v1.31.1) changed the wording of the duplicate-registration error from "is already attached to this model." to "is already registered in this model.". Because the second substring no longer matches, the exception was re-thrown instead of swallowed, aborting the model build.

Changes

• src/model.jl — the guards in variables_elements and variables_element_ext_data now accept both message variants for cross-version compatibility.
• test/test_utils.jl — the "Variable creation" testset now asserts either message variant.
• Bumped version 0.10.5 → 0.10.6 and added a NEWS.md entry.

Environment

• Julia 1.12.3
• JuMP 1.31.1

Related

• Issue: Test suite fails: variable name-collision guard no longer matches current JuMP error message #96