Handle attrs.asdict sets of instances

[lin-hongkuan]

Fixes #1180.

Summary

• allow recursive attrs.asdict() to finish when retained set-like collections contain attrs instances that become dictionaries
• fall back to a list only for set and frozenset when the converted values are unhashable
• cover both set and frozenset regressions in the next-gen attrs.asdict() tests

Tests

• uv run --python C:\Users\lin20\AppData\Local\Programs\Python\Python312\python.exe --with-editable . --group tests pytest tests\test_next_gen.py::TestAsDict -q
• uv run --python C:\Users\lin20\AppData\Local\Programs\Python\Python312\python.exe --with-editable . --group tests pytest tests\test_next_gen.py -q
• uv run --python C:\Users\lin20\AppData\Local\Programs\Python\Python312\python.exe --with-editable . --group tests pytest tests\test_funcs.py::TestAsDict -q
• uv run --python C:\Users\lin20\AppData\Local\Programs\Python\Python312\python.exe --with-editable . --group dev ruff check src\attr\_funcs.py tests\test_next_gen.py
• uv run --python C:\Users\lin20\AppData\Local\Programs\Python\Python312\python.exe --with-editable . --group dev ruff format --check src\attr\_funcs.py tests\test_next_gen.py
• git diff --check