feat(FN-3905): use merger session model for post-merge workflow steps

The merge implements post-merge model resolution, using the merger's own session model for executing post-merge prompt workflow steps instead of falling back to executor model defaults. Tests cover the resolution hierarchy, and documentation clarifies the precedence order for downstream consumers.

Fusion-Task-Id: FN-3905

packages/dashboard/src/routes/__tests__/custom-providers.test.ts

@@ -29,6 +29,7 @@ vi.mock("@fusion/core", async () => {
 
 vi.mock("@fusion/engine", () => ({
   createFnAgent: vi.fn(async () => ({ session: { state: { messages: [] }, prompt: vi.fn(), dispose: vi.fn() } })),
+  createResolvedAgentSession: vi.fn(async () => ({ session: { state: { messages: [] }, prompt: vi.fn(), dispose: vi.fn() } })),
   promptWithFallback: vi.fn(),
 }));
 

packages/dashboard/src/routes/register-chat-room-routes.ts

@@ -1,4 +1,5 @@
 import type { ChatAttachment, ChatRoomCreateInput, ChatRoomStatus, ChatRoomUpdateInput } from "@fusion/core";
+import { RoomReplyGenerationError } from "../chat.js";
 import { ApiError, badRequest, internalError, notFound } from "../api-error.js";
 import { rateLimit, RATE_LIMITS } from "../rate-limit.js";
 import type { ApiRoutesContext } from "./types.js";
@@ -265,6 +266,9 @@ export function registerChatRoomRoutes(ctx: ApiRoutesContext): void {
       res.status(201).json({ message: result.userMessage });
     } catch (err: unknown) {
       if (err instanceof ApiError) throw err;
+      if (err instanceof RoomReplyGenerationError) {
+        throw new ApiError(502, err.message);
+      }
       rethrowAsApiError(err, "Failed to create chat room message");
     }
   });