Allow bypass of automatic SSE conversion

**Is your feature request related to a problem? Please describe.**

The SDK automatically converts all streaming responses to SSE format even when already in SSE format. This prevents using custom SSE event types.

**Describe the solution you'd like**

Provide a way to customize SSE formatting when streaming responses. Either through a flag to bypass the automatic SSE conversion, or by detecting if the response if already in SSE format.

**Describe alternatives you've considered**

- Double-parsing on the client side.
- Including the event type in the data itself (though this makes efficient, type-safe parsing more difficult).

**Additional context**

The framework automatically detects generators/async generators and forces them through `_convert_to_sse()`.

Current behaviour:

```sse
data: "event: chunk\\ndata: {\\"bytes\\": \\"SGVsbG8gd29ybGQh\\"}\\n\\n"
```

Desired behaviour:

```sse
event: chunk
data: {"bytes": "SGVsbG8gd29ybGQh"}
```

Start by tracing the framework path that detects generators and async generators and calls `_convert_to_sse()`; compare how already-SSE responses are represented. Define the supported bypass or customization behavior, then verify that custom event types are preserved without double conversion for both streaming forms.

Written by the indexing model from the issue text.