Streaming responses silently turn unserialisable events into JSON strings of their Python repr

Start in runtime/app.py at _safe_serialize_to_json_string and trace its use in _stream_with_error_handling, then inspect convert_complex_objects in runtime/utils.py. Reproduce an entrypoint generator yielding a dictionary containing bytes and read the SSE data line; done means binary-containing events are not emitted as Python repr strings and instead follow one of the stated JSON-safe or explicit-error behaviors.

Describe the bug

When an entrypoint generator yields an object that json.dumps cannot serialise, BedrockAgentCoreApp._safe_serialize_to_json_string (runtime/app.py line 898 in 1.22.0) tries convert_complex_objects and then falls back to json.dumps(str(obj)). convert_complex_objects (runtime/utils.py) handles Pydantic models, dataclasses, dicts, lists, tuples and sets but not bytes, so any event that contains binary data anywhere in its tree reaches the third fallback. The SSE data: line then carries a JSON string holding the Python repr of the dictionary, for example "{'result': {'type': 'agent_result', 'message': {... b'rsn_...'}}}", rather than a JSON object or an error.

The client cannot tell this apart from a legitimate string event, and it cannot recover the original payload from the repr. In our case a supervisor agent consuming a sub-agent's stream did event.get("error") on what was now a str and failed with AttributeError: 'str' object has no attribute 'get', turning every affected turn into a user-facing failure. Nothing was logged on the producing side, because the fallback succeeds.

To Reproduce

1. Write a streaming entrypoint whose generator yields a dictionary containing a bytes value somewhere inside it. A real-world source is the AgentResult.to_dict() from Strands when the model returns a reasoningContent.redactedContent block, which OpenAI GPT-5.6 Luna on Bedrock does after tool calls. Any other bytes value in a yielded event, such as image or document source.bytes from a tool result, takes the same path.
2. Invoke the runtime and read the SSE stream.
3. Observe that the event arrives as data: "{'result': ...}" (a JSON string) instead of data: {"result": ...}.

Expected behavior

One of:

• binary values are converted to something JSON can carry (for example base64), or
• the event is replaced by an explicit error event, as _stream_with_error_handling already does for exceptions, and a warning is logged.

Either way the consumer should never receive a Python repr as the payload.

Additional context

• Versions: bedrock-agentcore 1.22.0 (latest on PyPI), Python 3.13, Strands 1.54.0, runtime container on Bedrock AgentCore.
• The final fallback that logs a warning only fires if json.dumps(str(obj)) itself fails, which it never does for a dictionary.
• Related: #198 (closed) reported bytes-in-message failing in the memory converter; this is the same class of input on the response-streaming path.
• Companion report about AgentResult.to_dict() not being JSON-serialisable: strands-agents/harness-sdk#4167.