fix(@nestjs/graphql): restore Timestamp scalar parsers in federation factory

[yogeshwaran-c]

Summary

• Extend the scalar restoration branch in GraphQLFederationFactory.overrideOrExtendResolvers to cover the built-in Timestamp scalar in addition to DateTime.
• Also copy serialize from the auto-generated scalar so the round-trip works regardless of which scalar shape the user picked.

Bug

After buildSubgraphSchema re-parses the printed SDL, the resulting executable schema has scalars with default (no-op) parsers. The override step only restored parseValue / parseLiteral for the DateTime scalar — DateTime happened to render correctly anyway because the default serializer hands a Date straight to JSON which produces an ISO string, matching GraphQLISODateTime.serialize's output.

When users opt into dateScalarMode: 'timestamp', the schema uses GraphQLTimestamp whose serialize returns value.getTime(). Without the restoration, that scalar fell back to the default serializer in the federated schema, so resolvers that return a Date ended up with the raw ISO string in the response instead of a numeric epoch.

// before — only DateTime is restored
} else if (isScalarType(type) && type.name === 'DateTime') {
  type.parseLiteral = autoGeneratedScalar.parseLiteral;
  type.parseValue   = autoGeneratedScalar.parseValue;
}

Fix

• Restore Timestamp alongside DateTime.
• Also restore serialize so the auto-generated scalar's full implementation comes through.

Test plan

• New e2e spec packages/apollo/tests/e2e/federation-timestamp-scalar.spec.ts boots an Apollo Federation app with dateScalarMode: 'timestamp', returns a Date(0) from a resolver, and asserts the response value is the numeric 0. Fails on master, passes after the fix.
• Existing tests/e2e/code-first-federation.spec.ts continues to pass.

[kamilmysliwiec]

lgtm