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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🔔 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!

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🔔 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!

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🔔 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!

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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!

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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.
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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.
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[ramakrishnap-nv]

/ok to test 6d5b75a

[ramakrishnap-nv]

/ok to test 575b1cc

[github-actions]

CI Test Summary

✅ All 0 test job(s) passed.