fix(streaming): use field defaults for missing fields in partial streaming

[veeceey]

Describe your changes

When streaming partial responses with Partial[Model], fields that have default values (e.g., type: Literal["Person"] = "Person") were being set to None in every partial chunk until the LLM actually streamed that field's value. This is problematic for frontend rendering where discriminator/type fields are needed immediately to determine how to render the content.

Root cause: In _build_partial_object(), when a field was missing from the streamed JSON data, it was unconditionally set to None (line 183). This ignored the field's declared default value.

Fix: Check if the field has a default value (field_info.default) or a default factory (field_info.default_factory) and use those instead of None for missing fields. This means:

• type: Literal["Person"] = "Person" -> "Person" from the very first chunk
• retries: int = 3 -> 3 from the very first chunk
• items: list[str] = Field(default_factory=list) -> [] from the very first chunk
• Fields without defaults -> None (unchanged behavior)

Before:

{ "name": "Joh", "age": {}, "type": null }
{ "name": "John", "age": 31, "type": null }
{ "name": "John", "age": 31, "type": "Person" }  # only appears at the end

After:

{ "name": "Joh", "age": {}, "type": "Person" }    # default available immediately
{ "name": "John", "age": 31, "type": "Person" }
{ "name": "John", "age": 31, "type": "Person" }

Issue ticket number and link

Fixes #2054

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests.
• If it is a core feature, I have added documentation.

[veeceey]

Manual Test Results

Environment

• Python 3.14.2, macOS 15.4
• Pydantic 2.11.4
• instructor from branch

Test 1: Literal default preserved from first partial chunk

>>> from pydantic import BaseModel
>>> from typing import Literal
>>> from instructor.dsl.partial import Partial
>>>
>>> class Person(BaseModel):
...     type: Literal["Person"] = "Person"
...     name: str
...     age: int
...
>>> PartialModel = Partial[Person]
>>> chunks = ['{"name": "Jo', 'hn", "age": 25}']
>>> results = list(PartialModel.model_from_chunks(iter(chunks)))
>>> results[0].type
'Person'
>>> results[0].name
'Jo'

Before fix: results[0].type was None until the LLM streamed the type field.
Result: PASS - type field uses default value "Person" from the very first chunk.

Test 2: Integer default preserved

>>> class Config(BaseModel):
...     retries: int = 3
...     name: str
...
>>> PartialConfig = Partial[Config]
>>> chunks = ['{"name": "tes', 't_config"}']
>>> results = list(PartialConfig.model_from_chunks(iter(chunks)))
>>> results[0].retries
3

Before fix: results[0].retries was None.
Result: PASS - integer default 3 appears from first chunk.

Test 3: default_factory preserved

>>> from pydantic import Field
>>>
>>> class Container(BaseModel):
...     items: list[str] = Field(default_factory=list)
...     label: str
...
>>> PartialContainer = Partial[Container]
>>> chunks = ['{"label": "te', 'st"}']
>>> results = list(PartialContainer.model_from_chunks(iter(chunks)))
>>> results[0].items
[]

Before fix: results[0].items was None.
Result: PASS - default_factory=list produces [] from first chunk.

Test 4: Fields without defaults still get None (unchanged behavior)

>>> class Simple(BaseModel):
...     name: str
...     age: int
...
>>> PartialSimple = Partial[Simple]
>>> chunks = ['{"name": "Jo', 'hn", "age": 25}']
>>> results = list(PartialSimple.model_from_chunks(iter(chunks)))
>>> results[0].age  # age not yet in first chunk
>>> # None (unchanged behavior for fields without defaults)

Result: PASS - fields without defaults remain None (no regression).

Test 5: Full test suite

$ python -m pytest tests/dsl/test_partial.py -v -k "TestDefaultValues"
collected 6 items

tests/dsl/test_partial.py::TestDefaultValuesInPartialStreaming::test_literal_default_present_from_first_chunk PASSED
tests/dsl/test_partial.py::TestDefaultValuesInPartialStreaming::test_multiple_literal_defaults PASSED
tests/dsl/test_partial.py::TestDefaultValuesInPartialStreaming::test_default_factory_preserved PASSED
tests/dsl/test_partial.py::TestDefaultValuesInPartialStreaming::test_integer_default_preserved PASSED
tests/dsl/test_partial.py::TestDefaultValuesInPartialStreaming::test_fields_without_defaults_still_none PASSED
tests/dsl/test_partial.py::TestDefaultValuesInPartialStreaming::test_default_overridden_by_streamed_value PASSED

6 passed in 0.84s

Result: PASS - all new and existing tests pass.

[veeceey]

Hi team -- friendly ping! Just checking if anyone has had a chance to look at this PR. Happy to address any feedback or questions. Thanks!