fix(streaming): add direct type-map fallback for raw dict events (fixes #941)

[Zelys-DFKH]

Problem

The accumulate_event function in the streaming message parser fails with:

Unexpected event runtime type, after deserialising twice - {...} - <class 'dict'>

This occurs when SSE events like content_block_delta are received as raw dicts instead of properly deserialized Pydantic models.

Root Cause

Pydantic's union discriminator can fail silently in certain versions or configurations. Instead of raising an error, construct_type_unchecked falls back to returning the raw dict—making the failure invisible until the type check fails later.

Solution

Add a fallback type-map lookup when the dict is returned. This approach:

1. Defines _RAW_EVENT_TYPE_MAP and _BETA_RAW_EVENT_TYPE_MAP dictionaries that map event type strings to their corresponding Pydantic model classes
2. When a dict is returned, uses the dict's type field to look up the correct event class
3. Constructs the model directly via model_construct(**raw_dict)

The fallback catches the silent failure early. Well-formed events now construct correctly, and malformed events still raise the original error—so we get both robustness and clear error messages.

Changes

• Added imports for all RawMessageStreamEvent union members to both _messages.py and _beta_messages.py
• Added _RAW_EVENT_TYPE_MAP and _BETA_RAW_EVENT_TYPE_MAP dictionaries
• Updated error handling in accumulate_event to implement the fallback before raising an error

Testing

This fix has been tested to handle:

• content_block_delta events arriving as raw dicts
• Proper fallback to existing error when dict is malformed or type is unknown
• Backward compatibility with existing working implementations