Harden Daum-Huang particle-flow input validation

[FlorianPfaff]

Summary

Fix three correctness issues in the Daum-Huang Gaussian particle-flow helpers:

• reject nonsymmetric covariance matrices instead of silently symmetrizing them before PSD validation;
• reject complex-valued particles, weights, measurements, Jacobians, matrices, and schedules before NumPy float coercion can discard imaginary components;
• validate delta_lambda as a finite nonnegative scalar in the public Gaussian bridge APIs.

Regression coverage

Adds focused tests for nonsymmetric state/measurement covariances, complex NumPy inputs, and invalid bridge exponents. Existing valid affine-bridge and EDH/LEDH behavior is unchanged.

This is independent of the currently open replay-validation, hyperhemispherical-mixture, and von-Mises-subclass PRs.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Max errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no		no	8.36s
✅ JSON	prettier	7	0	0		0	0.7s
✅ JSON	v8r	7		0		0	4.17s
✅ MARKDOWN	markdownlint	68	0	0		0	2.2s
✅ MARKDOWN	markdown-table-formatter	68	0	0		0	0.41s
✅ PYTHON	black	2009	24	0		0	64.82s
✅ PYTHON	isort	2009	40	0		0	3.06s
✅ REPOSITORY	betterleaks	yes		no		no	1.33s
✅ REPOSITORY	checkov	yes		no		no	39.97s
✅ REPOSITORY	git_diff	yes		no		no	0.14s
✅ REPOSITORY	secretlint	yes		no		no	119.5s
✅ REPOSITORY	syft	yes		no		no	3.67s
✅ REPOSITORY	trivy-sbom	yes		no		no	2.47s
✅ YAML	prettier	11	0	0		0	0.77s
✅ YAML	v8r	11		0		0	9.78s
✅ YAML	yamllint	11		0		0	1.07s

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