test routing null matrix validation

[aycsi]

Description

DataModel.add_cost_matrix routes through validate_matrix, which is expected to reject matrices containing NULL values before solve-time logic. This change adds a focused negative test to lock down that contract and prevent silent regressions during future validation.

Summary

• Add test_dist_mat_null to routing validation coverage.
• Cover the NULL input path for cost matrices in DataModel.add_cost_matrix.

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[github-actions]

🔔 Hi @anandhkb, this pull request has had no activity for 7 days. Please update or let us know if it can be closed. Thank you!

If this is an "epic" issue, then please add the "epic" label to this issue.
If it is a PR and not ready for review, then please convert this to draft.
If you just want to switch off this notification, then use the "skip inactivity reminder" label.

[github-actions]

🔔 Hi @anandhkb, this pull request has had no activity for 7 days. Please update or let us know if it can be closed. Thank you!

If this is an "epic" issue, then please add the "epic" label to this issue.
If it is a PR and not ready for review, then please convert this to draft.
If you just want to switch off this notification, then use the "skip inactivity reminder" label.

[github-actions]

🔔 Hi @anandhkb, this pull request has had no activity for 7 days. Please update or let us know if it can be closed. Thank you!

If this is an "epic" issue, then please add the "epic" label to this issue.
If it is a PR and not ready for review, then please convert this to draft.
If you just want to switch off this notification, then use the "skip inactivity reminder" label.