feat(dataclass): add asdict, astuple, and match_args support

[junrushao]

Summary

Adds three stdlib-parity features to tvm_ffi.dataclasses:

1. asdict(obj, *, dict_factory=dict) — recursively converts a @py_class / @c_class instance to a plain Python dict. FFI containers (Array, List) recurse into list; (Map, Dict) recurse into dict, yielding JSON-ready output.
2. astuple(obj, *, tuple_factory=tuple) — the tuple analogue of asdict, with the same recursion rules.
3. match_args: bool = True parameter on @py_class and @c_class — sets cls.__match_args__ to the tuple of positional __init__ field names (init=True and not kw_only), enabling Python 3.10+ match statements. Skipped when the class body already defines __match_args__.

Semantics follow CPython's dataclasses module: asdict/astuple raise TypeError for types and non-dataclass values; kw-only fields (via field(kw_only=True), decorator-level kw_only=True, or the KW_ONLY sentinel) are excluded from __match_args__.

Design notes

• _is_ffi_dataclass_instance filters FFI container instances (Array, List, Map, Dict) from FFI dataclass instances — both share __tvm_ffi_type_info__, so the container isinstance-check runs first during recursion.
• _set_match_args walks the TypeInfo.parent_type_info chain in parent-first order, matching the order of the auto-generated __init__ signature.
• Recursion uses an _ATOMIC_TYPES frozenset (mirroring stdlib dataclasses._ATOMIC_TYPES) for the fast path on immutable leaves.

Test plan

• uv run pytest tests/python/ — 2184 passed, 38 skipped, 3 xfailed
• pre-commit run --files <touched files> — all hooks pass (ruff, ty, format)
• New coverage in tests/python/test_dataclass_common.py:
  • TestAsdict (15 tests): basic, nested, inheritance, FFI Array/List/Map/Dict recursion, dict_factory, result independence, error paths
  • TestAstuple (10 tests): basic, nested, recursion, tuple_factory, error paths
  • TestMatchArgs (11 tests): defaults, init=False, kw_only (field / decorator / KW_ONLY sentinel), inheritance order, match_args=False opt-out, user-defined __match_args__ override, c_class basic and inheritance

[gemini-code-assist]

Code Review

This pull request introduces asdict and astuple utility functions for FFI dataclasses, mirroring the standard library's dataclasses functionality. It also adds support for __match_args__ in both @c_class and @py_class decorators to enable structural pattern matching. The review feedback correctly identifies a bug in the recursive handling of collections.defaultdict within both asdict and astuple, where the code incorrectly checks the class instead of the instance for the default_factory attribute.