Add WithoutBroadcasting attribute to skip oversized stream events

[pushpak1300]

Currently there isn't a clean way to stop BroadcastAgent from broadcasting every StreamEvent. Tool results from data-heavy tools (listing records, search, email bodies) routinely blow past the 10KB WebSocket frame limit on Reverb/Pusher, which throws a BroadcastException and fails the whole queued job. Folks have been working around it by subclassing the job and overriding broadcastOnQueue (see #174).

This PR adds a class attribute so an affected agent can opt out of broadcasting specific event types:

#[WithoutBroadcasting(ToolResult::class, ToolCall::class)]
class SearchAgent implements Agent, HasTools
{
    use Promptable;
}

The named events simply never hit the websocket; the frontend loads tool data from agent_conversation_messages after the stream completes, same as the existing workaround. No attribute means everything broadcasts as before, so nothing changes for the majority who never hit the limit. It applies to both the queued (broadcastOnQueue) and synchronous (broadcast/broadcastNow) paths.

Went with an attribute to match the existing #[MaxSteps]/#[Timeout] style and avoid widening the Agent contract or threading a new param through three broadcast signatures. Open to renaming if you'd prefer something else.

Fixes #174

[WendellAdriel]

Is this expected to be withholds or should be withHolds?

[pushpak1300]

i think for simplicity changed to excludes.

[WendellAdriel]

LGTM 💪