feat(16-04): update route tests and MCP tests for multi-user userId

- All 8 route test files destructure { db, userId } from createTestDb()
- All route test middleware sets c.set("userId", userId)
- MCP tools.test.ts passes userId to all registerXTools(db, userId) calls
- MCP tools.test.ts passes userId to getCollectionSummary(db, userId)
- Added 4 cross-user isolation tests for MCP tools (items, item by ID, threads, collection summary)
- OAuth test db type annotation updated for new createTestDb return shape
- Images test now uses createTestDb with userId context

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/routes/images.test.ts

@@ -1,8 +1,15 @@
 import { describe, expect, test } from "bun:test";
 import { Hono } from "hono";
 import { imageRoutes } from "../../src/server/routes/images";
+import { createTestDb } from "../helpers/db.ts";
 
+const { db, userId } = createTestDb();
 const app = new Hono();
+app.use("*", async (c, next) => {
+	c.set("db", db);
+	c.set("userId", userId);
+	await next();
+});
 app.route("/api/images", imageRoutes);
 
 describe("POST /api/images/from-url", () => {