fix: convert Bedrock streaming error to APIStatusError

[xodn348]

Fixes #1477

What

When Bedrock returns a non-200 status code inside an SSE event (e.g. internalServerException), the AWSEventStreamDecoder now yields an "error" ServerSentEvent instead of raising a raw ValueError. The existing Stream.__stream__() error handler catches this event and converts it to a proper APIStatusError, matching the behavior of the non-Bedrock streaming path.

Why

Previously, _parse_message_from_event raised ValueError on non-200 event status codes. This was problematic because:

• ValueError is a generic exception that doesn't carry HTTP status info
• Users cannot catch it with the SDK's standard error hierarchy (except APIStatusError, except BadRequestError, etc.)
• It bypasses the SDK's existing error-handling infrastructure entirely

How

Changed _parse_message_from_event to return ServerSentEvent | None instead of str | None. On non-200 status codes, it returns a ServerSentEvent(event="error", data=...) with the Bedrock error details formatted as an Anthropic API error body. The iter_bytes/aiter_bytes methods now yield these events directly, allowing the Stream.__stream__() error handler to convert them to APIStatusError via _client._make_status_error().

Verification

• Build: pass
• Tests: pass (41 passed — 6 new + 35 existing)
• Lint: pass (ruff check)
• Type check: pass (pyright — 0 errors)