### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

## Description

When a Bedrock guardrail hard-blocks a streamed response, `async_post_call_streaming_iterator_hook` raises an `HTTPException` inside the async generator body. By the time the guardrail runs, the HTTP 200 header has already been written to the client — the exception cannot be converted to a 4xx response. Instead it propagates up, truncating the SSE stream mid-flight. The client receives a partial or malformed response with no indication of why.

## Expected behaviour

The client receives a well-formed SSE error frame followed by `[DONE]`:

data: {"error": {"message": "Violated guardrail policy", "code": 400, "type": "guardrail_violation"}}

data: [DONE]

## Actual behaviour

The `HTTPException` escapes the generator. The stream is truncated and the client has no structured error to parse.

## Root cause

`async_post_call_streaming_iterator_hook` has no `try/except` around the `make_bedrock_api_request` / `asyncio.gather` calls. Both the parallel INPUT+OUTPUT path (`should_validate_input=True`) …