feat(validate): accept dataclass instances as input

[Oaklight]

Summary

• validate() now accepts dataclass instances directly — no need to manually convert to dict first
• Instances are auto-converted via dataclasses.fields() + getattr() at each nesting level, so nested dataclass trees are validated recursively
• Cross-type validation works: a dataclass instance can be validated against a TypedDict schema (and vice versa)
• Works with both regular and slots=True dataclasses
• Previously, passing a dataclass instance would raise ValidationError: Expected dict, got <ClassName>

Implementation

2-line change in _validate_struct_fields: check if value is a dataclass instance, convert via {f.name: getattr(value, f.name) for f in dataclasses.fields(value)} before the existing isinstance(value, dict) guard. Each nesting level does its own lazy conversion. This approach:

• Returns a fresh dict (no __dict__ aliasing — safe to mutate)
• Works with slots=True dataclasses (no __dict__ needed)
• Only surfaces declared fields (excludes post-init artifacts)

Test plan

• 10 new tests: flat/nested instance validation, invalid field detection, cross-schema (DC→TypedDict), list-of-DC, default fields, slots=True, no-mutation safety
• All 129 existing tests pass (no regressions)
• All 19 benchmark tests pass
• Pre-commit hooks (ruff, ty, complexipy) pass

Docs

Bilingual docs updated in docs_en/docs_zh branches:

• Module docs: new "Dataclass Instance Validation" section with examples
• API reference: updated data parameter description
• Changelog: added entry under [Unreleased]

[Oaklight]

Review feedback addressed:

1. __dict__ aliasing — switched from vars() to {f.name: getattr(value, f.name) for f in dataclasses.fields(value)}, returns a fresh dict that won't mutate the original instance.
2. slots=True support — dataclasses.fields() + getattr() works regardless of whether __dict__ exists.

Both issues covered by new tests (test_instance_slots, test_instance_no_mutation). All 129 tests pass, pre-commit clean. Bilingual docs updated with corrected implementation details and pushed to docs_en/docs_zh branches.