heartbeatIntervalMs missing from ApolloServerPluginSubscriptionCallback options

### Issue Description

Apollo Server docs for `ApolloServerPluginSubscriptionCallback` mention a configurable
`heartbeatIntervalMs`, but the current TypeScript interface does not expose this option.
As a result, users cannot configure heartbeat behavior in a supported or type-safe way.

Docs reference:
https://github.com/apollographql/apollo-server/blob/main/docs/source/api/plugin/subscription-callback.mdx#heartbeatintervalms

HTTP-callback subscriptions are sensitive to timing and latency between the router/gateway
and the subgraph.

Without a supported way to configure the subgraph’s heartbeat interval:
- we cannot compensate for event-loop jitter or network latency
- valid subscriptions may be terminated despite healthy communication

### Link to Reproduction

https://github.com/mario-jerkovic/apollo-server-subscription-callback-plugin-issue

### Reproduction Steps

Run:
```bash
yarn install
yarn build
```

Actual `tsc` fails with and error

```bash
error TS2353: Object literal may only specify known properties, and 'heartbeatIntervalMs' does not exist in type 'ApolloServerPluginSubscriptionCallbackOptions'.
```

Locate the TypeScript definition of ApolloServerPluginSubscriptionCallbackOptions and compare it with the heartbeatIntervalMs option documented in the linked API page. Use the reproduction repository and run yarn install, then yarn build; done means the example type-checks without the TS2353 error.

Written by the indexing model from the issue text.