fix: route Bedrock SSE error events to the error handler

[xodn348]

Fixes #1472

What

The Bedrock AWSEventStreamDecoder previously hardcoded all SSE events as event="completion", causing error payloads (e.g. rate_limit_error) from Bedrock to be deserialized as RawMessageStartEvent with message=None instead of being routed to the existing error handler. This resulted in AttributeError: 'NoneType' object has no attribute 'model' crashes during streaming.

This fix inspects the decoded event payload and emits event="error" for events with "type": "error", allowing the existing error handling path in Stream.__stream__() to raise a proper APIStatusError.

Verification

• Build: pass
• Tests: pass (29 passed — streaming + bedrock decoder tests)
• Lint: pass (ruff check)
• Type check: pass (pyright 0 errors)

[sevakva]

Hey @xodn348
I've opened a PR #1479 that implements a broader fix for bedrock errors, including error codes mapping to anthropic errors. That handles other streaming errors and propagates exact errors to the user