Fix UnboundLocalError on single-clause npm version ranges

[arpitjain099]

While parsing native node-semver ranges I hit an internal crash on some wildcard inputs.

NpmVersionRange.from_native() sends tokens like >=1.x and *.x through get_npm_version_constraints_from_semver_npm_spec(). That helper only assigns anyof_constraints inside the isinstance(clause, (AnyOf, AllOf)) branch. When NpmSpec(...).clause.simplify() reduces to a single node-semver Range clause instead of an AnyOf/AllOf, execution falls straight through to the return, so a raw UnboundLocalError leaks out of the parser:

>>> from univers.version_range import NpmVersionRange
>>> NpmVersionRange.from_native(">=1.x")
UnboundLocalError: local variable 'anyof_constraints' referenced before assignment

>=1.x, *.x, >1.x and <=2.x all trip it, while ranges that expand to two bounds (1.x, 0.x, ~1.x, 1.2.x - 2.0.0) were fine because they simplify to an AllOf.

The fix flattens the branch and adds an explicit case for the single Range clause, building one VersionConstraint from its operator and target the same way get_allof_constraints does per constraint. Any other unexpected clause type now raises InvalidVersionRange rather than the old (unreachable) ValueError. Multi-clause handling is unchanged.

After the change:

>>> str(NpmVersionRange.from_native(">=1.x"))
'vers:npm/>=1.0.0'
>>> str(NpmVersionRange.from_native("*.x"))
'vers:npm/>=0.0.0'

Added parametrized tests in tests/test_version_range.py covering the previously-crashing single-clause inputs and confirming the multi-clause ranges still produce the same output. The full tests/test_version_range.py suite passes.