Union federation fails when __typename is in query

**Demo repository**: https://github.com/TheAschr/federation-bug-demo

I have a specific situation where federation of a union fails. here is my query:

```gql
{
accounts {
id
user {
... on StandardUser {
id
film {
id
__typename
}
}
... on AdminUser {
id
film {
id
review {
id
}
}
}
}
}
}
```

Here is the response I get:

```gql
{
"errors": [
{
"message": "Field \"reviewId\" was not found in response.",
"extensions": {
"code": "INTERNAL_SERVER_ERROR",
"exception": {
"stacktrace": [
"Error: Field \"reviewId\" was not found in response.",
" at executeSelectionSet (C:\\Users\\me\\Documents\\github\\federation-demo\\node_modules\\@apollo\\gateway\\dist\\executeQueryPlan.js:217:27)",
" at executeSelectionSet (C:\\Users\\me\\Documents\\github\\federation-demo\\node_modules\\@apollo\\gateway\\dist\\executeQueryPlan.js:236:51)",
" at C:\\Users\\me\\Documents\\github\\federation-demo\\node_modules\\@apollo\\gateway\\dist\\executeQueryPlan.js:121:36",
" at Array.forEach ()",
" at executeFetch (C:\\Users\\me\\Documents\\github\\federation-demo\\node_modules\\@apollo\\gateway\\dist\\executeQueryPlan.js:120:18)",
" at executeNode (C:\\Users\\me\\Documents\\github\\federation-demo\\node_modules\\@apollo\\gateway\\dist\\executeQueryPlan.js:82:23)",
" at executeNode (C:\\Users\\me\\Documents\\github\\federation-demo\\node_modules\\@apollo\\gateway\\dist\\executeQueryPlan.js:73:33)",
" at executeNode (C:\\Users\\me\\Documents\\github\\federation-demo\\node_modules\\@apollo\\gateway\\dist\\executeQueryPlan.js:56:46)",
" at processTicksAndRejections (internal/process/task_queues.js:97:5)",
" at async Object.executeQueryPlan (C:\\Users\\me\\Documents\\github\\federation-demo\\node_modules\\@apollo\\gateway\\dist\\executeQueryPlan.js:22:27)",
" at async Object.ApolloGateway.executor (C:\\Users\\me\\Documents\\github\\federation-demo\\node_modules\\@apollo\\gateway\\dist\\index.js:109:30)"
]
}
}
}
],
"data": {
"accounts": [
{
"id": "1",
"user": {
"id": "1",
"film": {
"id": "1",
"review": null
}
}
},
{
"id": "2",
"user": {
"id": "2",
"film": {
"id": "1",
"__typename": "Film"
}
}
}
]
}
}
```

I believe it must be a bug because when I remove __typename the query completes:

```gql
{
accounts {
id
user {
... on StandardUser {
id
film {
id
}
}
... on AdminUser {
id
film {
id
review {
id
}
}
}
}
}
}
```

Reproduce the query against the linked federation-bug-demo repository and compare it with the version that omits __typename. Start from the executeSelectionSet stack trace in @apollo/gateway/dist/executeQueryPlan.js; done means the union query completes without the missing reviewId error and preserves the expected response.

Written by the indexing model from the issue text.