fix(v2): derive Iterable name from union members instead of "Union"

[Sanjays2402]

What this fixes

When building an Iterable response model for a union task type, the generated class name collapses to IterableUnion for both typing.Union[A, B] and PEP 604 A | B, instead of a member-derived name like IterableAOrB.

Root cause

In instructor/v2/dsl/iterable.py, the naming logic was:

# Handle `Union[A, B]` / `A | B` task types.
# `types.UnionType` does not have `__name__`, so fall back to a stable name.
task_name = getattr(subtask_class, "__name__", None)
if task_name is None and get_origin(subtask_class) in _UNION_ORIGINS:
    members = get_args(subtask_class)
    task_name = "Or".join(getattr(m, "__name__", str(m)) for m in members)
if task_name is None:
    task_name = str(subtask_class)

The comment's assumption is incorrect: a union type's __name__ is "Union", not None. (And on modern Python, A | B / types.UnionType normalizes to typing.Union, whose __name__ is also "Union".) So getattr(..., "__name__", None) returns "Union", the if task_name is None guard is never entered, the member-joining branch is skipped, and every union becomes IterableUnion.

This affected both union spellings:

prepare_response_model(list[A | B]).__name__          # -> "IterableUnion"  (want IterableAOrB)
prepare_response_model(List[Union[A, B]]).__name__    # -> "IterableUnion"  (want IterableAOrB)

The fix

Detect a union origin first, and only fall back to __name__/str() for non-union types:

if get_origin(subtask_class) in _UNION_ORIGINS:
    members = get_args(subtask_class)
    task_name = "Or".join(getattr(m, "__name__", str(m)) for m in members)
else:
    task_name = getattr(subtask_class, "__name__", None) or str(subtask_class)

Result:

Input	Before	After
list[A | B]	IterableUnion	IterableAOrB
List[Union[A, B]]	IterableUnion	IterableAOrB
list[A | B | C]	IterableUnion	IterableAOrBOrC
list[User] (non-union)	IterableUser	IterableUser (unchanged)

Tests

The existing test_list_of_model_pipe_union_generates_clean_name was failing on main (it asserts IterableAOrB but got IterableUnion) — this fix makes it pass. I also added two regression tests for the typing.Union form and a three-way union.

pytest tests/dsl tests/core -k "not test_patch_completes and not test_apatch_completes"
135 passed, 3 skipped

(The two deselected tests instantiate OpenAI() and require an API key — unrelated to this change; they fail identically on main.)

ruff check passes on both changed files. black reports one pre-existing reformat elsewhere in iterable.py that is present on main too and is left untouched.

Type of change

• Bug fix (non-breaking) — improves generated class names for union iterables; non-union names are unchanged.