fix(python): propagate JSON Schema default values to generated class

[schani]

Bug

JSON Schema default values on object properties were silently dropped by
quicktype: they never appeared on the generated Python class, and were never
used as a fallback when the corresponding key was missing from the input
dict.

Repro (from #1415):

{
  "definitions": {
    "record:Metadata": {
      "type": "object",
      "required": ["Name"],
      "additionalProperties": false,
      "properties": {
        "Name": { "default": "Hello", "type": "string" }
      }
    }
  },
  "$ref": "#/definitions/record:Metadata"
}

node dist/index.js --lang python --src-lang schema meta.schema

produced name: str with no default, and from_dict did
from_str(obj.get("Name")) with no fallback — so a document missing "Name"
would raise an AssertionError instead of falling back to "Hello".

Root cause

The default keyword was never read out of the JSON Schema input at all —
nothing in quicktype-core's attribute-producer pipeline stored it on the
type graph, so no renderer could ever see it.

Fix

• Added packages/quicktype-core/src/attributes/DefaultValues.ts: a new
  TypeAttributeKind (propertyDefaultValues) plus a
  defaultValuesAttributeProducer that reads each object property's
  default (when present) off the JSON Schema and attaches it to the
  object type.
• Wired the new producer into JSONSchemaInput's attribute-producer list.
• In the Python renderer (PythonRenderer.ts, JSONPythonRenderer.ts):
  • Emit the default as a Python literal on the dataclass/plain-class field
    (name: str = "Hello"), including support for None, numbers, bools,
    lists, and dicts (mutable defaults use dataclasses.field(default_factory=...)
    where appropriate).
  • Use the default as the fallback argument to obj.get(...) in
    from_dict so round-tripping a document that omits the key still
    produces the schema default.
  • Adjusted sortClassProperties (and the shared ConvenienceRenderer
    base method signature it overrides) so properties with defaults are
    sorted after non-defaulted ones, keeping the generated class valid.
• Scoped to the Python target, per the issue; other languages are
  unaffected (only Python overrides the touched ConvenienceRenderer
  method).

Test coverage

Added test/inputs/schema/default-values.schema with two defaulted string
properties, a .1.json sample that omits one of them, and a
.1.out.defaults.json expected-output file (matching the repo's existing
per-feature .out.<feature>.json fixture convention) that has the default
filled in. Registered a new "defaults" language feature and enabled it for
Python in test/languages.ts.

Verification

• npm run build passes cleanly.
• Manually confirmed the fix against the issue's exact repro: generated
  Python now has name: str = "Hello" and
  obj.get("Name", "Hello").
• Confirmed the new fixture fails on the pre-fix code (Expected property Greeting not found, exit 1) and passes after the fix.
• QUICKTEST=true FIXTURE=schema-python script/test — all 72 schema-python
  fixtures pass, including the new one.
• QUICKTEST=true FIXTURE=python script/test — all 62 JSON python fixtures
  pass (no regressions from the shared ConvenienceRenderer signature
  change).
• Other Python renderer modes spot-checked manually (plain classes,
  --pydantic-base-model) and all correctly emit the default.
• CI will run the full fixture matrix for other languages/configurations.

Fixes #1415

🤖 Generated with Claude Code

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