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:

yarn install
yarn build

Actual tsc fails with and error

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