fix(NODE-5937): restore non-string Map key guard in serializer

Commit a22387a dropped the `typeof key === 'string'` guard from the key
validation block on the assumption that keys are always strings. That holds
for plain objects and arrays, but Map keys can be non-string. Without the
guard, a non-string Map key with checkKeys:true reached `key.includes('.')`
and threw a TypeError from a different site than the baseline.

Restoring the guard makes behavior byte-identical to the pre-regression
implementation: non-string Map keys (unsupported by design, NODE-3027) fail
in encodeUTF8Into exactly as before, regardless of checkKeys. The Set lookup
optimization is unaffected — it remains gated by `key[0] === '$'`.

Author: tadjik1

src/parser/serializer.ts

@@ -587,7 +587,7 @@ export function serializeInto(
       value = value.toBSON();
     }
 
-    if (!frame.isArray && !(key[0] === '$' && ignoreKeys.has(key))) {
+    if (!frame.isArray && typeof key === 'string' && !(key[0] === '$' && ignoreKeys.has(key))) {
       if (regexp.test(key)) {
         throw new BSONError('key ' + key + ' must not contain null bytes');
       }