⬆️ Bump sinon-chai to support chai@6

Dependabot bumped `chai` to v6, but CI then fails at `npm install`:

    npm error ERESOLVE unable to resolve dependency tree
    npm error peer chai@"^4.0.0" from sinon-chai@3.7.0

`sinon-chai@3.7.0` only peer-depends on `chai@^4`, so it can't coexist
with `chai@6`.

This change bumps `sinon-chai` to `^4.0.1`, whose peer range is
`chai@^5 || ^6`, which clears the conflict. `sinon-chai@4` ships as ESM
with a default export, so under Node's `require(esm)` interop
`require('sinon-chai')` resolves to the module namespace rather than the
plugin function; we now reach for `.default` so `chai.use()` gets the
function it expects.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: alecgibson

package.json

@@ -26,7 +26,7 @@
     "sharedb": "^6.0.0",
     "sharedb-mingo-memory": "^5.0.0",
     "sinon": "^22.0.0",
-    "sinon-chai": "^3.7.0"
+    "sinon-chai": "^4.0.1"
   },
   "scripts": {
     "lint": "./node_modules/.bin/eslint .",

test/test_mongo_middleware.js

@@ -1,7 +1,7 @@
 var async = require('async');
 var sinon = require('sinon');
 var chai = require('chai');
-chai.use(require('sinon-chai'));
+chai.use(require('sinon-chai').default);
 var expect = chai.expect;
 var ShareDbMongo = require('..');
 var mongodb = require('./../mongodb');