Fix get() fallback for unset model fields

[PEliet]

SubscriptableBaseModel exposes mapping-style helpers, but get() used
hasattr() to decide whether a field was present. Pydantic exposes unset
optional fields as attributes with their class default, so an unset field such
as Message.content returned None instead of the fallback passed to get().
This also disagreed with __contains__ and __getitem__, which already treat
the same field as absent.

Use the model's existing membership semantics in get(). The regression test
covers both sides of the distinction: an unset optional field returns the
fallback, while a field explicitly set to None still returns None.

Validation:

• pytest tests/test_type_serialization.py::test_subscriptable_model_get_returns_default_for_unset_field -q
• pytest ollama/_types.py --doctest-modules -q
• ruff check ollama/_types.py tests/test_type_serialization.py
• ruff format --check ollama/_types.py tests/test_type_serialization.py
• pytest ollama tests -q -k "not generate_images and not create_blob and not image_serialization_path and not image_serialization_string_path" (91 passed, 8 deselected)

The eight deselected tests use open NamedTemporaryFile handles in ways that
fail on Windows. This is the existing issue #676 and is unrelated to this
change.

Prepared with assistance from OpenAI Codex.