fix: guard ParsedMessageStopEvent.message type to suppress Pydantic warnings

[lizradway]

model_dump() on ParsedMessageStopEvent emits PydanticSerializationUnexpectedValue warnings on every streamed response because the unresolved generic in message: ParsedMessage[ResponseFormatT] confuses Pydantic’s serializer at runtime (see Pydantic ref)

The fix is the same TYPE_CHECKING guard already used for ParsedContentBlockStopEvent.content_block right below, it was just missed here:

if TYPE_CHECKING:
    message: ParsedMessage[ResponseFormatT]
else:
    message: ParsedMessage

It looks like this inconsistency was introduced in ad56677 and only shows up at serialization time rather than typechecking or validation, likely explaining why this was not caught

Reproduced on every SDK version from 0.84.0 through 0.113.0 with pydantic 2.9–2.13.

Testing

Verified with the following across multiple SDK/pydantic version combinations:

import warnings
from anthropic.types.parsed_message import ParsedTextBlock, ParsedMessage
from anthropic.lib.streaming._types import ParsedMessageStopEvent
from anthropic._models import build
import anthropic

parsed_msg = ParsedMessage.construct(
    id="msg_test", type="message", role="assistant",
    content=[ParsedTextBlock.construct(type="text", text="Hello!")],
    model="claude-sonnet-4-20250514", stop_reason="end_turn",
    usage=anthropic.types.Usage.construct(input_tokens=10, output_tokens=5),
)
event = build(ParsedMessageStopEvent, type="message_stop", message=parsed_msg)

with warnings.catch_warnings(record=True) as caught:
    warnings.simplefilter("always")
    event.model_dump()

assert len(caught) == 0  # 12 sub-warnings before fix, 0 after

Also confirmed:

• ruff check / ruff format pass
• pyright / mypy pass with 0 errors
• tests/test_streaming.py — 23 tests pass

Related: #1175, #1422
Resolves: #1715