Support incremental delivery (@defer/@stream) in gateway executor path

Use case

Apollo Server added incremental delivery support for @defer/@stream in #8148, but this only works for the direct execution path (executeIncrementally). The gateway executor path (internals.gatewayExecutor) always wraps the result as { singleResult: result }, which means incremental delivery results from a gateway are silently ignored.

When using @apollo/gateway or any custom GatewayExecutor implementation that supports @defer, the incremental results cannot flow through to the client because:

1. GatewayExecutionResult is typed as plain ExecutionResult — gateways can't return incremental results
2. requestPipeline.ts doesn't check for initialResult/subsequentResults in the gateway branch

Proposed change

The runtime fix in requestPipeline.ts is small — check if the gateway result contains initialResult and route it through the existing formatErrorsInSubsequentResults handlers, matching the pattern in the direct execution branch:

// packages/server/src/requestPipeline.ts
} else if (internals.gatewayExecutor) {
  const result = await internals.gatewayExecutor(
    makeGatewayGraphQLRequestContext(requestContext, server, internals),
  );
  if ('initialResult' in result) {
    return {
      initialResult: result.initialResult,
      subsequentResults:
        'pending' in result.initialResult
          ? formatErrorsInSubsequentResultsAlpha9(
              result.subsequentResults as
AsyncIterable<GraphQLExperimentalSubsequentIncrementalExecutionResultAlpha9>,
            )
          : formatErrorsInSubsequentResultsAlpha2(
              result.subsequentResults,
            ),
    };
  } else {
    return { singleResult: result };
  }

The part I'm less sure about is how to best define the incremental delivery types in @apollo/server-gateway-interface. GatewayExecutionResult needs to become a union that includes incremental result types (both Alpha2 and Alpha9), but there are different approaches for how detailed those type definitions should be. I have a working prototype that mirrors the incrementalDeliveryPolyfill.ts structure, but I'd welcome input on the preferred approach.

I have a prototype with tests on my fork: https://github.com/Re-cool/apollo-server/tree/feat/gateway-incremental-delivery