Handle matrix sequence conversion runtime errors

[FlorianPfaff]

Summary

• treat backend RuntimeError during outer matrix-sequence conversion like the existing TypeError and ValueError cases
• fall back to the established per-entry matrix normalization path
• add a focused regression that simulates a backend unable to stack the outer sequence

Root cause

AbstractSmoother._normalize_matrix_sequence() first tries to convert the complete input sequence with the active backend's asarray(). Some backends raise RuntimeError, rather than TypeError or ValueError, when a Python sequence cannot be converted as one dense array. The vector-sequence counterpart already handles this backend behavior, but the matrix-sequence path did not. Consequently, valid per-step matrix sequences could fail before the existing per-entry fallback was reached.

Impact

Matrix sequence normalization now behaves consistently with vector sequence normalization and remains usable when a backend cannot stack the outer Python sequence directly. Inputs that are genuinely invalid still fail through the existing validation path.

Validation

• branch is two commits ahead of current main and zero behind
• production change is limited to one additional caught exception
• regression monkeypatches the backend conversion to raise RuntimeError only for the outer sequence and verifies both matrices are normalized through the fallback

The full backend test matrix is delegated to GitHub Actions.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	22.72s
✅ JSON	prettier	7	0	0	0	1.15s
✅ JSON	v8r	7		0	0	3.63s
✅ MARKDOWN	markdownlint	68	0	0	0	1.92s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.74s
✅ PYTHON	black	1490	182	0	0	90.52s
✅ PYTHON	isort	1490	328	0	0	2.49s
✅ REPOSITORY	betterleaks	yes		no	no	2.52s
✅ REPOSITORY	checkov	yes		no	no	55.9s
✅ REPOSITORY	gitleaks	yes		no	no	14.19s
✅ REPOSITORY	git_diff	yes		no	no	0.31s
✅ REPOSITORY	secretlint	yes		no	no	65.01s
✅ REPOSITORY	syft	yes		no	no	5.22s
✅ REPOSITORY	trivy-sbom	yes		no	no	6.14s
✅ REPOSITORY	trufflehog	yes		no	no	32.89s
✅ YAML	prettier	11	0	0	0	0.68s
✅ YAML	v8r	11		0	0	10.84s
✅ YAML	yamllint	11		0	0	0.65s

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