Fix discrete-state validation regressions

[FlorianPfaff]

Summary

Fix two correctness regressions in src/pyrecest/filters/discrete_state.py that are already covered by regression tests on main:

• reject NaN and +inf log-emission values in scaled_emissions, while preserving -np.inf as zero likelihood mass;
• restore strict positive-integer validation for sticky_mode_transition_matrix / mode_transition_matrix, including rejection of booleans, floats, vector-valued arrays, and timedeltas while accepting NumPy integer scalars.

Why this matters

NaN and +inf were silently mapped to zero mass because all non-finite emissions shared the same handling path. This can hide upstream numerical failures and change inference results without an exception.

The mode-count validation was previously fixed in #4812 but was lost when the discrete-state implementation was later consolidated into discrete_state.py, leaving the regression tests behind.

Validation

Focused validation against the existing regression cases passes locally for:

• tests/filters/test_discrete_state_nonfinite_emissions.py
• tests/filters/test_discrete_state_mode_count_validation.py

The patch changes only one source file (11 additions, 2 deletions).

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Max errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no		no	4.28s
✅ JSON	prettier	7	0	0		0	0.36s
✅ JSON	v8r	7		0		0	3.39s
✅ MARKDOWN	markdownlint	68	0	0		0	1.26s
✅ MARKDOWN	markdown-table-formatter	68	0	0		0	0.45s
✅ PYTHON	black	2009	23	0		0	37.16s
✅ PYTHON	isort	2009	39	0		0	1.67s
✅ REPOSITORY	betterleaks	yes		no		no	0.67s
✅ REPOSITORY	checkov	yes		no		no	22.97s
✅ REPOSITORY	git_diff	yes		no		no	0.09s
✅ REPOSITORY	secretlint	yes		no		no	57.47s
✅ REPOSITORY	syft	yes		no		no	1.64s
✅ REPOSITORY	trivy-sbom	yes		no		no	1.46s
✅ YAML	prettier	11	0	0		0	0.37s
✅ YAML	v8r	11		0		0	11.98s
✅ YAML	yamllint	11		0		0	0.6s

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