fix: support default {} on freeform object schemas

Summary

A freeform object schema (type: object, additionalProperties: true,
no declared properties) inside an anyOf could not carry a
default: {}. The parser rejected the default, which silently dropped
the enclosing schema and every endpoint that referenced it. This PR
accepts the empty-dict default and generates a working empty-container
initializer.

Problem

A schema declared as type: object with additionalProperties: true
and no declared properties (often appearing inside an anyOf with
null) could not carry a default: {}. ModelProperty.convert_value
rejected the default with ModelProperty cannot have a default value,
which propagated up as a warning and silently dropped the enclosing
schema and every endpoint that referenced it.

Minimal reproduction

components:
  schemas:
    ModelWithFreeformDefault:
      type: object
      properties:
        extras:
          anyOf:
            - type: object
              additionalProperties: true
            - type: "null"
          default: {}

Before this PR: ModelWithFreeformDefault is missing from the
generated client; the only signal is a warning on stderr.

Fix

Three coordinated changes in model_property.py and union.py:

1. ModelProperty.convert_value accepts {} on a freeform model
   and emits a ClassName() constructor call. When
   required_properties has not yet been populated, the check falls
   back to the raw schema (data.properties, data.allOf,
   data.anyOf, data.oneOf) so a referenced schema still gets the
   correct decision before its properties have been processed.

2. Import promotion in ModelProperty. A default value forces the
   generated client to import the inner model at runtime rather than
   only under TYPE_CHECKING, otherwise the default initializer would
   NameError at class-definition time. get_imports and
   get_lazy_imports are updated so inner-property imports are
   promoted when the property has a non-None default.

3. Import promotion in UnionProperty. The same promotion applies
   to unions that carry a default — every inner property's lazy
   imports become runtime imports so the union's default expression
   resolves at class-definition time.

Tests

• Functional test:
  end_to_end_tests/functional_tests/generated_code_execution/test_freeform_object_defaults.py
  — uses the inline spec above, asserts the model is instantiable
  and that the default extras is the empty container.
• Unit tests in
  tests/test_parser/test_properties/test_model_property.py:
  • test_get_imports_with_default and test_get_lazy_imports_with_default
    cover the import-promotion branches.
  • test_convert_value and test_convert_value_unprocessed cover
    convert_value's new accept/reject decisions, including the
    pre-processing fallback path.

Local verification

pdm install
pdm run ruff format . --check
pdm run ruff check .
pdm mypy --show-error-codes
pdm test_with_coverage

All green on Python 3.14. 100% coverage on both changed files
(model_property.py, union.py) when measured across the full
upstream suite. Golden-record snapshot tests pass without
regeneration — the import-promotion paths are only reachable when a
model has a non-None default, which previously errored, so no
existing snapshot can depend on the old behaviour.

Risk

• ModelProperty.convert_value changes from a @classmethod to an
  instance method. A grep across openapi_python_client/, tests/,
  and end_to_end_tests/ finds zero ModelProperty.convert_value(...)
  class-level call sites; every caller uses prop.convert_value(...)
  via the PropertyProtocol. The signature change is invisible to
  existing call sites.
• Acceptance is conservative: only {} is accepted, and only on
  a model that is genuinely freeform (no properties, allOf,
  anyOf, or oneOf). Any other default value still returns
  PropertyError with the original message, so models with required
  fields can't slip through with an empty default.
• Snapshot impact: confirmed none. Snapshot tests pass without
  pdm regen.

Related

Companion PR: #1448 — a second independent fix for the
title-collision case surfaced from the same downstream spec. The two
PRs share no code paths and can land in either order.