Fix GNN pairwise-cost input validation

[FlorianPfaff]

Summary

• coerce pairwise_cost_matrix through the active backend before validating its shape, so documented array-like inputs such as Python lists work
• tighten measurement-covariance shape validation so malformed 1-D or wrong-depth covariance inputs raise the intended ValueError instead of leaking indexing errors
• add focused GNN regression tests for list pairwise costs and invalid covariance shapes

Rationale

GlobalNearestNeighbor.find_association() documents pairwise_cost_matrix as array-like, but the validator accessed .shape before coercion. Passing a plain nested Python list therefore failed with AttributeError before the association logic could use it. The same validation block also accessed cov_mats_meas.shape[2] whenever ndim != 2, so 1-D malformed covariance input raised IndexError rather than the intended validation error.

Validation

• Connector compare confirms this branch is ahead of current main by 2 commits and behind by 0.
• Changes are limited to src/pyrecest/filters/global_nearest_neighbor.py and tests/filters/test_global_nearest_neighbor_pairwise_costs.py.
• Full local pytest was not run because the sandbox cannot resolve GitHub; PR CI should run the focused GNN tests and full matrix.

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
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✅ MARKDOWN	markdown-table-formatter	66	0	0	0	0.83s
✅ PYTHON	black	759	2	0	0	39.87s
✅ PYTHON	isort	759	2	0	0	2.55s
✅ REPOSITORY	checkov	yes		no	no	47.81s
✅ REPOSITORY	gitleaks	yes		no	no	18.14s
✅ REPOSITORY	git_diff	yes		no	no	0.08s
✅ REPOSITORY	secretlint	yes		no	no	22.63s
✅ REPOSITORY	syft	yes		no	no	4.43s
✅ REPOSITORY	trivy-sbom	yes		no	no	6.97s
✅ REPOSITORY	trufflehog	yes		no	no	23.11s
✅ YAML	prettier	11	0	0	0	0.83s
✅ YAML	v8r	11		0	0	11.37s
✅ YAML	yamllint	11		0	0	0.55s

Detailed Issues

⚠️ MARKDOWN / markdownlint - 1 error

.github/pull_request_template.md:1 error MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## Summary"]

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