fix(bedrock): raise APIStatusError instead of ValueError for non-200 stream events

[nileshpatil6]

Fixes #1477

Problem

AWSEventStreamDecoder._parse_message_from_event raises a raw ValueError when Bedrock returns a non-200 status code inside the event stream (e.g. internalServerException, throttlingException). This happens independently of the HTTP response status -- Bedrock signals these errors through the event stream protocol with the event's own status_code field.

The raw ValueError is uncatchable as a standard SDK error, gives no structured information to the caller, and surfaces as a confusing crash.

Fix

Replace the ValueError with a proper APIStatusError subclass constructed from the event's status_code and parsed body:

• 429 -> RateLimitError
• >= 500 -> InternalServerError
• 400 -> BadRequestError
• anything else -> APIStatusError

A synthetic httpx.Response is built from the event's status code so the exception contract (response, status_code, message) is fully satisfied.

Before / After

# Before -- uncatchable, no structured info
ValueError: Bad response code, expected 200: {'status_code': 400, 'headers': {':exception-type': 'internalServerException', ...}, 'body': b'...'}

# After -- catchable, structured
anthropic.InternalServerError: The system encountered an unexpected error during processing. Try your request again.

Users can now handle these by type:

from anthropic import InternalServerError, RateLimitError

async with client.messages.stream(...) as stream:
    try:
        async for chunk in stream:
            ...
    except RateLimitError:
        # retry
    except InternalServerError:
        # log and retry