fix: convert Bedrock streaming error SSE events to APIStatusError

[xodn348]

Fixes #1477

What

The Bedrock stream decoder (AWSEventStreamDecoder) raises a raw ValueError when it encounters a non-200 status code in an SSE event (e.g., internalServerException with status 400/500). This bypasses the SDK's error handling, so users see ValueError: Bad response code, expected 200: ... instead of a proper APIStatusError subclass like InternalServerError or BadRequestError.

This fix changes _parse_message_from_event to emit a ServerSentEvent(event="error") for non-200 status codes instead of raising ValueError. The existing error handling in Stream.__stream__ / AsyncStream.__stream__ already catches event="error" SSE events and converts them to the appropriate APIStatusError subclass via _make_status_error.

Verification

• Build: pass
• Tests: pass (14 passed, including 2 new tests for the fix)
• Lint: pass (ruff)
• Type check: pass (pyright, 0 errors)