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

**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.

Bắt đầu trong runtime/app.py tại _safe_serialize_to_json_string và lần theo cách nó được sử dụng trong _stream_with_error_handling, sau đó kiểm tra convert_complex_objects trong runtime/utils.py. Tái hiện một trình tạo entrypoint trả về một dictionary chứa bytes và đọc dòng dữ liệu SSE; hoàn thành khi các event chứa dữ liệu nhị phân không được phát ra dưới dạng chuỗi repr của Python mà thay vào đó tuân theo một trong các hành vi JSON-safe hoặc lỗi tường minh đã nêu.

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