[swf1996120]

Chemical repository testing study: this PR fixes a bug found by an LLM-assisted boundary and invariant-testing pass. I reproduced and minimized the behavior locally, searched existing reports, and added focused regression tests.

Fixes #5538

Timestep.dimensions previously stored impossible or non-finite unit-cell angles without validation, allowing invalid triclinic vectors to propagate into periodic geometry calculations. The patch requires all angles to be finite and strictly between 0° and 180°, while preserving the existing no-unit-cell sentinel and length checks.

Validation: Cython source and focused tests compile; regression coverage added in test_timestep_api.py.

[read-the-docs-community]

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[IAlibay]

@swf1996120 - we require everyone to fill in the PR template especially the AI use section and having the developer certificate of origin declaration.

The former is made clear in our AI policy: https://github.com/MDAnalysis/mdanalysis/blob/develop/AI_POLICY.md#0-ai-usage-must-always-be-declared

Given this failure to follow policy, and your excessive pull requests across the open source ecosystem (e.g. openmm/openmm#5559 (comment)), I am going to block and close this PR.