linter: add regression test for outer local used in object assert (#778)

[bejaratommy]

Problem

Issue #778 reports that jsonnet-lint incorrectly flags a local variable as unused when it is only referenced inside an object-level assert expression:

local myvar = 'xxxxx';

{
  assert std.length(myvar) == 5 : 'yyyyy',
  some_field: 'zzzzz',
}

The linter was incorrectly reporting:

Unused variable: myvar

Root Cause & Fix

The underlying fix was already introduced in #723 (commit e0c6a9e), which added traversal of DesugaredObject.Asserts during variable-usage analysis in findVariablesInObject. After that fix, object-level assert expressions are walked with the correct scope — including variables from outer local bindings — so occurrences are properly recorded.

This PR

Adds a dedicated regression test case covering the exact scenario reported in #778: an outer (non-object-level) local used exclusively inside an object assert. The expected golden output is empty, confirming no spurious "unused variable" error is emitted.

Closes #778

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.