fix: resolve subscripted generics in _build_discriminated_union_meta (pydantic v1)

[ohxh]

Problem

_build_discriminated_union_meta in _models.py accesses variant.__fields__ directly on each Union variant. When a variant is a subscripted generic like ParsedBetaTextBlock[ResponseFormatT] (from ParsedBetaContentBlock), it is a typing._GenericAlias — not a class — so .__fields__ raises AttributeError on Python 3.12.

This crashes streaming (client.beta.messages.stream()) because accumulate_event calls construct_type(type_=ParsedBetaContentBlock, ...) on every content_block_start event, and when pydantic v1's validate_type fails to parse the block, it falls through to _build_discriminated_union_meta which hits this bug.

Only the pydantic v1 code path is affected. Pydantic v2 uses _extract_field_schema_pv2 which handles subscripted generics correctly.

Fix

Use get_origin(variant) or variant before accessing .__fields__, matching the pattern the SDK's own is_basemodel_type already uses (line 460):

# Before (crashes on subscripted generics):
field_info = cast(..., variant.__fields__).get(discriminator_field_name)

# After:
origin = get_origin(variant) or variant
field_info = cast(..., origin.__fields__).get(discriminator_field_name)

Repro

# With pydantic v1 installed:
client = anthropic.Anthropic()
with client.beta.messages.stream(
    model="claude-sonnet-4-6",
    max_tokens=1024,
    messages=[{"role": "user", "content": "hello"}],
    betas=["..."],
) as stream:
    for event in stream:  # AttributeError: __fields__
        pass

AttributeError: __fields__
  File "anthropic/lib/streaming/_beta_messages.py", line 133, in __stream__
    self.__final_message_snapshot = accumulate_event(...)
  File "anthropic/lib/streaming/_beta_messages.py", line 481, in accumulate_event
    construct_type(type_=ParsedBetaContentBlock, value=event.content_block.to_dict())
  File "anthropic/_models.py", line 549, in construct_type
    discriminator = _build_discriminated_union_meta(union=type_, meta_annotations=meta)
  File "anthropic/_models.py", line 689, in _build_discriminated_union_meta
    field_info = cast(..., variant.__fields__).get(discriminator_field_name)
  File "typing.py", line 1186, in __getattr__
    raise AttributeError(attr)

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