Type checking issues with getClient.mutate()

Hi there,

We've been using this project and it has been excellent for the most part - greatly appreciated.

There is one issue we've been running into - `getClient().mutate()` calls don't automatically type-check input variables. `getClient()` is the RSC implementation from the docs:

```ts
export const { getClient } = registerApolloClient(() => {
// ...
```

With our `graphql-codegen`-generated types, the return types all Just Work, but variables are only type-checked if we explicitly set both types in the `mutate` generic arguments. Otherwise, it infers the type from the provided variables irrespective of the GraphQL schema, causing run-time errors rather than build-time.

Here's an example. **The return type is correctly set.**

```ts
return getClient().mutate({
mutation: MESSAGE_MARK_READ,
variables: {
input: {
messageId,
test: true, // <-- this is not in the mutation input in GQL, but no type error
},
},
});
```

After adding types as `mutate

Start by comparing the RSC getClient().mutate() typing described in the issue with the hook-based useMutation and the graphql-codegen-generated types. Reproduce both cases: an extra input field and an input object assigned to a variable; done means invalid variables produce build-time errors without requiring explicit generic types.

Written by the indexing model from the issue text.