fix(streaming): add direct type-map fallback for raw dict events

[gambletan]

Summary

Fixes #941 — content_block_delta SSE events fail to deserialize with "Unexpected event runtime type, after deserialising twice" in some environments.

Root cause

accumulate_event in _messages.py (and _beta_messages.py) receives events from Stream[RawMessageStreamEvent]. In certain environments (older pydantic versions, or edge cases in the Annotated-union discriminator path), construct_type_unchecked silently returns the raw dict instead of a typed BaseModel. The previous code retried once then raised TypeError.

Fix

Add a third-tier fallback: if both the initial isinstance check and the construct_type_unchecked retry fail to produce a BaseModel, look up the event's "type" field in an explicit dict[str, type[BaseModel]] map and call model_construct directly. This guarantees that any well-formed event dict (including content_block_delta) is always promoted to the correct typed object.

The same fix is applied to the beta messages streaming path (_beta_messages.py).

Changes

• src/anthropic/lib/streaming/_messages.py — import concrete event classes, add _RAW_EVENT_TYPE_MAP, add fallback in accumulate_event
• src/anthropic/lib/streaming/_beta_messages.py — same for the beta path
• tests/lib/streaming/test_messages.py — regression test that passes a raw dict event directly to accumulate_event and asserts it succeeds

Test plan

• New regression test test_accumulate_event_handles_raw_dict_content_block_delta passes raw dict events to accumulate_event and asserts they are handled correctly
• All existing streaming tests pass (tests/lib/streaming/ — 26 tests)

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