fix: resolve VRS model class via getattr in from-VRS translation

[developer-rpai]

Bug

Translating FROM VRS (a dict VRS object) via _Translator._from_vrs crashed with TypeError: 'module' object is not subscriptable. The code subscripted models (the ga4gh.vrs.models module) with var["type"].

Root cause

In src/ga4gh/vrs/extras/translator.py, _from_vrs did model = models[var["type"]]. Modules are not subscriptable, so any VRS dict input crashed. The surrounding try/except KeyError was meant to return None for unknown types, but the TypeError was never caught.

Fix

Resolve the model class with getattr(models, var["type"], None) and return None gracefully when the type is unknown, preserving the original intent. Scope is the crash only; the broader translator redesign discussed in the thread is left to maintainers.

Verification

• Reproduced the crash on pristine upstream main before the fix: translate_from(vrs_dict, fmt="vrs") raised TypeError at translator.py line 170.
• Added regression test test_from_vrs_dict in tests/extras/test_allele_translator.py: a valid VRS dict now translates to an Allele without TypeError; unknown types, non dict input, and missing type all return None.
• After the fix the new test passes, and the valid dict round trips to a models.Allele.

What I could not verify

• The full existing translator test suite cannot run green in this sandbox: 20 tests in test_allele_translator.py and 7 in test_cnv_translator.py fail identically before and after this change because they need live network access to uta.biocommons.org and a local seqrepo REST server, which are unavailable here. No new failures were introduced by this change.
• CI on this PR has not been observed; I am not claiming it is green.

Closes #489