Reject non-finite finite-difference steps in EKF trackers

[FlorianPfaff]

Bug

MEMSOEKFTracker and EKFSplineTracker converted finite_difference_step to float and only rejected values <= 0. Because comparisons with NaN are false and +Inf is positive, both trackers accepted non-finite step sizes. Those values then propagate directly into finite-difference perturbations and can produce non-finite Jacobians/Hessians and poison subsequent filter updates.

Fix

• require finite_difference_step to be finite as well as positive in both trackers
• keep behavior unchanged for valid positive finite steps
• add focused regressions for NaN, +Inf, and -Inf

Scope

The production diff is limited to the two constructor guards plus math.isfinite; the new test file only exercises the input contract.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Max errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no		no	7.5s
✅ JSON	prettier	7	0	0		0	0.84s
✅ JSON	v8r	7		0		0	4.32s
✅ MARKDOWN	markdownlint	68	0	0		0	2.22s
✅ MARKDOWN	markdown-table-formatter	68	0	0		0	0.4s
✅ PYTHON	black	2027	8	0		0	67.98s
✅ PYTHON	isort	2027	11	0		0	3.04s
✅ REPOSITORY	betterleaks	yes		no		no	1.36s
✅ REPOSITORY	checkov	yes		no		no	36.65s
✅ REPOSITORY	git_diff	yes		no		no	0.19s
✅ REPOSITORY	secretlint	yes		no		no	109.12s
✅ REPOSITORY	syft	yes		no		no	3.27s
✅ REPOSITORY	trivy-sbom	yes		no		no	2.88s
✅ YAML	prettier	11	0	0		0	0.69s
✅ YAML	v8r	11		0		0	17.09s
✅ YAML	yamllint	11		0		0	1.23s

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