Fix just() conversion for container fields of dataclass types (#858)

[martinez-hub]

Summary

Fixes #858. just failed to convert a dataclass to a config when one of its fields is annotated with a container of a dataclass type (e.g. dict[str, Inner] or list[Inner]).

import hydra_zen as hz
from dataclasses import dataclass

@dataclass
class Inner:
    x: int = 0

@dataclass
class Outer:
    outer: dict[str, Inner]

hz.to_yaml(hz.just(Outer(outer={"a": Inner(x=1)})))
# omegaconf.errors.ValidationError: Invalid type assigned:
# Builds_Inner is not a subclass of Inner. value: <class 'types.Builds_Inner'>

Root cause

just recursively converts the dataclass values into Builds_Inner configs, but the field's annotation keeps the original element type (dict[str, Inner]). OmegaConf type-checks container elements before instantiation and rejects the Builds_Inner config since it isn't a subclass of Inner.

_retain_type_info already broadens a field's annotation to Any when its value is a structured config — but it only inspected the top-level value, not configs nested inside a container. A second gap: mutable container defaults are stored via default_factory, so the value reaching that check was MISSING.

Fix

• _value_contains_builds(value) — detects a structured config nested (possibly deeply) inside a list/tuple/mapping.
• _resolve_field_default(field) — resolves a field's default_factory so the real container value can be inspected.
• _retain_type_info now broadens the annotation to Any when the value is a container holding a structured config.

Broadening to Any only loosens OmegaConf validation — it never tightens an annotation, so previously-working configs are unaffected.

Tests

Added to tests/test_dataclass_conversion.py:

• test_just_on_container_field_of_typed_dataclasses — dict[str, Inner], list[Inner], and nested dict[str, list[Inner]], each round-tripped through to_yaml → instantiate.
• Unit tests for both new helpers.

Verification

• Full test suite passes locally (test_launch/test_with_hydra_submitit/test_third_party excluded — they run in CI).
• 100% coverage on the changed lines.
• pyright basic: 0 errors on the changed source.
• ruff format --check, ruff check, and autoflake all clean (ruff 0.14.1).
• Changelog entry added under a new 0.16.1 section.

[martinez-hub]

The failing tests (3.9) and test-minimum-dependencies checks here are unrelated to this change. They're the pre-existing, order-dependent flaky failure tracked in #574 — MissingConfigException: In 'hydra/config': Could not find 'hydra/sweeper/basic' in tests/test_launch/test_validation.py — which #876 fixes at its root cause.

All other checks pass, including coverage, run-pyright, and tests on 3.10–3.14. Once #876 is merged, this branch can be rebased onto main and CI will go fully green.