GeezerVison

Fixes #171

VectorField, HalfVectorField and SparseVectorField pass form and serializer input straight to the text parsers. A malformed value made form.is_valid() raise ValueError, and a value with the wrong number of dimensions passed validation and only failed on save() with a DataError from Postgres. In both cases the request ended in a 500.

Changes:

• to_python turns parse errors into ValidationError (Enter a valid vector. and the half/sparse variants).
• Each field adds a validator, the way max_length adds MaxLengthValidator. It checks that the value converts to the vector type and, when dimensions is set, that the length matches (Expected 3 dimensions, not 2.). Since it is in validators, DRF's ModelField also runs it, and ArrayField(VectorField(dimensions=3)) reports the bad item.
• Valid input behaves as before. Fields without dimensions skip the length check.

Tests: new cases in tests/test_django.py for full_clean and the three model forms, covering malformed input and wrong dimensions. The full suite, mypy and ty check pass on my fork's CI run (Postgres with pgvector 0.8.7).