fix: improve error message for invalid factorization constraint set (#293)

[ArpanC6]

Fixes #293

The error message thrown when a factorization constraint set is invalid was hard to interpret. The raw BoundedBitSetTuple matrix printed in the error gave users no real guidance on what went wrong or how to fix it.

I added a short hint to the error message explaining what a valid constraint set actually requires so users have something actionable to go on rather than just a matrix of booleans.

Hint

A valid constraint set requires that each interface belongs to exactly one factorization group. In the constraint matrix, each column must have exactly one non-zero entry per group. Make sure all variables appearing in your model are included in your factorization constraint.

Changes

• src/plugins/variational_constraints/variational_constraints_engine.jl - added hint to error message
• test/plugins/variational_constraints/variational_constraints_engine_tests.jl - added test asserting the hint is present

Test plan

• New test passes locally.
• Existing tests still pass.

[ArpanC6]

Hi @bvdmitri, the CI workflows are showing "Action required" for my PR #335. I see this is a fork-based PR from ArpanC6:fix/293-constraint-error. Could you please approve the workflow runs so CI can start? The change is small: a descriptive hint in the invalid factorization constraint set error message plus a regression test. Thanks!

[bvdmitri]

Why is this necessary for updating the error message?

[bvdmitri]

We dont have CHANGELOG in this repository and this PR should not add it

[bvdmitri]

This PR also seems to be a copy of https://github.com/ReactiveBayes/GraphPPL.jl/pull/303/changes

[ArpanC6]

@bvdmitri thanks for the review. I've rebased the branch and addressed all three points

1. src/graph_engine.jl - the change was unrelated to this PR. It came in from the rebase onto upstream/main. It's no longer in the diff (the comment above is now marked Outdated).
2. CHANGELOG.md - removed. I mistakenly added it during the rebase - this repository doesn't have a CHANGELOG.
3. PR fix: support Colon() in middle of indices for NodeLabel (#275) #303 duplicate - the e84d5c5 and 284c117 commits were upstream commits that got pulled in during the rebase. I've rebased so the branch now contains only the two commits that are actually part of this change:
   • d66c0cd fix: improve error message for invalid factorization constraint set (Possibly can improve constraint set error #293)
   • 0b3221e test: assert the invalid factorization constraint error message includes a hint

The diff is now just the error-message hint and its regression test. Could you please re-review and approve the workflow runs so CI can start? Thanks..