Fix missing array dimensions in QUBin sparse constructor calls

[Copilot]

• Understand the issue: sparse(I, J, V) call at line 47 in src/library/format/qubin/parser.jl is missing the array dimensions m, n, and similarly sparsevec(li, lv) at line 41 is missing the dimension n
• Verify existing tests pass
• Fix the issue by adding dimension n to both sparsevec and sparse calls in the parser
• Add a test case that specifically validates the fix (model with 3 variables where variable 3 has no linear/quadratic terms)
• Run tests to verify the fix works correctly (610 tests pass)
• Run code review (no issues found)
• Run CodeQL security check (no issues - Julia not analyzed)
• Add julia-actions/cache to CI workflow to speed up builds

Original prompt

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This section details on the original issue you should resolve

<issue_title>Array dimensions missing in sparse(I, J, V, [m, n]) constructor</issue_title>
<issue_description>https://github.com/JuliaQUBO/QUBOTools.jl/blob/46493c498b70e94f2fa49daf0e38a278fea33d09/src/library/format/qubin/parser.jl#L47C1-L47C48</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Array dimensions missing in sparse(I, J, V, [m, n]) constructor #55

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[bernalde]

@copilot Address this:
[julia-buildpkg] Caching of the julia depot was not detected
Consider using julia-actions/cache to speed up runs https://github.com/julia-actions/cache To ignore, set input ignore-no-cache: true

[Copilot]

@copilot Address this:
[julia-buildpkg] Caching of the julia depot was not detected
Consider using `ju...

Added julia-actions/cache@v2 to the CI workflow in commit 831547f. This will cache the Julia depot to speed up subsequent workflow runs.