Firestore: CollectionReference.withConverter(converter).add(data) invokes converter.toFirestore twice

The `toFirestore` method of converter is called multiple times when adding a new document to a collection which can cause issues if the object is mutated as part of the conversion (it seems reasonable to assume it should only be called once).

Similar to this client-side issue: https://github.com/firebase/firebase-js-sdk/issues/3742

#### Environment details

- OS: macOS 14.6.1
- Node.js version: 22.4.0
- npm version: pnpm 9.7.1
- `@google-cloud/firestore` version: 7.9.0

#### Steps to reproduce

Use the `.add` method on a collection ref with a converter, mutating the object:

```
const testConverter = {
toFirestore(test) {
test.hash = Blob.fromBase64String(test.hash)
return test
},
fromFirestore(snapshot, options) {
const data = snapshot.data(options)!;
data.hash = data.hash.toBase64()
return data
}
};

const ref = await firestore
.collection(`test`)
.withConverter(testConverter)
.add({ name: 'test', hash: 'some-base64-string-that-we-want-to-store-as-a-blob' })
```

The converter would work fine for regular `set`, and `update` methods, but `.add` causes `.toFirestore` to be called twice. Although it's a good idea for the converters to use immutable patterns which would avoid this, it's an easy mistake to just mutate the existing object which can cause runtime errors or data corruption and if nothing else is wasted extra work.