fix: remove hardcoded disableBlockStreaming to honor agent config for TUI

The chat.send handler was hardcoding disableBlockStreaming: true, which
overrode the agents.defaults.blockStreamingDefault config setting. This
caused TUI to ignore the blockStreamingDefault config and always stream
token-by-token, overwriting previous content.

Now the blockStreamingDefault config is respected for TUI/webchat just
like other channels.

Fixes #19643

src/gateway/server-methods/chat.ts

@@ -898,7 +898,6 @@ export const chatHandlers: GatewayRequestHandlers = {
           runId: clientRunId,
           abortSignal: abortController.signal,
           images: parsedImages.length > 0 ? parsedImages : undefined,
-          disableBlockStreaming: true,
           onAgentRunStart: (runId) => {
             agentRunStarted = true;
             const connId = typeof client?.connId === "string" ? client.connId : undefined;