feat(FN-3817): fix chat delta spacing in legacy streaming API

Fixes chat streaming delta spacing to preserve proper whitespace, adding regression tests for legacy chat stream handlers and the createChatStreamHandlers factory. A changeset was created for the `@runfusion/fusion` package.

Fusion-Task-Id: FN-3817

.changeset/fn-3817-chat-delta-spacing.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Preserve whitespace at SSE delta boundaries in dashboard chat streaming so streamed multi-sentence assistant responses render `. ` correctly between sentences in ChatView and QuickChatFAB.

packages/dashboard/app/api/__tests__/legacy-chat-stream.test.ts

@@ -139,6 +139,65 @@ describe("streamChatResponse SSE parser", () => {
     expect(onError).toHaveBeenCalledWith("Timed out waiting for first response event");
     vi.useRealTimers();
   });
+
+  it.each([
+    {
+      name: "leading-space second delta",
+      chunks: [
+        "event: text\n",
+        "data: \"Hello.\"\n\n",
+        "event: text\n",
+        "data: \" World.\"\n\n",
+      ],
+    },
+    {
+      name: "space-trailing first delta",
+      chunks: [
+        "event: text\n",
+        "data: \"Hello. \"\n\n",
+        "event: text\n",
+        "data: \"World.\"\n\n",
+      ],
+    },
+    {
+      name: "empty delta between spaced chunks",
+      chunks: [
+        "event: text\n",
+        "data: \"Hello.\"\n\n",
+        "event: text\n",
+        "data: \"\"\n\n",
+        "event: text\n",
+        "data: \" World.\"\n\n",
+      ],
+    },
+    {
+      name: "chunk boundary mid-json of second delta",
+      chunks: [
+        "event: text\ndata: \"Hello.\"\n\nevent: text\ndata: \"",
+        " World.\"\n\n",
+      ],
+    },
+    {
+      name: "chunk boundary inside leading space on data line",
+      chunks: [
+        "event: text\ndata: \"Hello.\"\n\nevent: text\ndata: \" ",
+        "World.\"\n\n",
+      ],
+    },
+  ])("preserves whitespace at SSE delta boundaries: $name", async ({ chunks }) => {
+    vi.spyOn(globalThis, "fetch").mockResolvedValue(new Response(createChunkedStream(chunks), { status: 200 }));
+
+    const textChunks: string[] = [];
+
+    streamChatResponse("s-1", "hi", {
+      onText: (data) => textChunks.push(data),
+      onError: vi.fn(),
+    });
+
+    await vi.waitFor(() => {
+      expect(textChunks.join("")).toBe("Hello. World.");
+    });
+  });
 });
 
 describe("attachChatStream", () => {

packages/dashboard/app/api/legacy.ts

@@ -8580,6 +8580,8 @@ export function streamChatResponse(
             currentEvent = line.slice(6).trim();
           } else if (line.startsWith("data:")) {
             const value = line.slice(5);
+            // Strip only the optional SSE protocol delimiter-space after `data:`.
+            // Payload whitespace (including JSON-string leading spaces) must stay verbatim.
             currentDataLines.push(value.startsWith(" ") ? value.slice(1) : value);
           } else if (line === "") {
             const currentData = currentDataLines.join("\n");
@@ -8780,6 +8782,8 @@ export function attachChatStream(
             currentEvent = line.slice(6).trim();
           } else if (line.startsWith("data:")) {
             const value = line.slice(5);
+            // Strip only the optional SSE protocol delimiter-space after `data:`.
+            // Payload whitespace (including JSON-string leading spaces) must stay verbatim.
             currentDataLines.push(value.startsWith(" ") ? value.slice(1) : value);
           } else if (line === "") {
             const currentData = currentDataLines.join("\n");

packages/dashboard/app/hooks/__tests__/createChatStreamHandlers.test.ts

@@ -0,0 +1,49 @@
+import { describe, expect, it, vi } from "vitest";
+import { createChatStreamHandlers } from "../createChatStreamHandlers";
+
+describe("createChatStreamHandlers", () => {
+  it("preserves whitespace across streamed text deltas", () => {
+    vi.useFakeTimers();
+
+    let text = "";
+    const onDone = vi.fn();
+    const onError = vi.fn();
+    const cancelStreamingFlushesRef = { current: null } as { current: (() => void) | null };
+
+    const { handlers } = createChatStreamHandlers({
+      sessionId: "s-1",
+      tempUserMessageId: "temp-1",
+      setStreamingText: (value) => {
+        text = typeof value === "function" ? value(text) : value;
+      },
+      setStreamingThinking: vi.fn(),
+      setStreamingToolCalls: vi.fn(),
+      cancelStreamingFlushesRef,
+      onDone,
+      onError,
+    });
+
+    handlers.onText("Hello.");
+    handlers.onText("");
+    handlers.onText(" World.");
+
+    vi.advanceTimersToNextTimer();
+
+    expect(text).toBe("Hello. World.");
+
+    handlers.onDone({ messageId: "m-1" });
+    expect(onDone).toHaveBeenCalledWith({
+      messageId: "m-1",
+      message: undefined,
+      accumulated: {
+        text: "Hello. World.",
+        thinking: "",
+        toolCalls: [],
+        fallbackInfo: undefined,
+      },
+    });
+    expect(onError).not.toHaveBeenCalled();
+
+    vi.useRealTimers();
+  });
+});