Prevent merging of fragment fields into selection tree if fragment field merging is disabled

[AnthonyMDev]

Currently, when fragment field merging is disabled we are preventing the fields from being merged into the MergedSelections while traversing the built EntitySelectionTree. However, the performance issues described in apollographql/apollo-ios#3434 are actually caused during the merging of the fragment fields into the EntitySelectionTrees during the building of the operations.

This PR prevents the fragment selection trees from being merged into the EntitySelectionTree while building the operations if fragment field merging is disabled.

This means that the entity trees are actually incomplete, but they still work for the purposes of Codegen in this scenario. Incomplete EntitySelectionTrees would cause selection set initializers to be invalid. But this is not a problem because disabling fragment field merging is already incompatible with selection set initializers (and we have a validation step to ensure those options are not used together).

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Configuration

{
  "repoOverrides": {
    "apollographql/apollo-ios-dev@main": {
      "remote": {
        "owner": "apollographql",
        "repo": "apollo-ios-dev",
        "branch": "fragment-field-merging-disabled-performance"
      }
    }
  }
}

Name	Link
Commit	4e76056
Preview	https://www.apollographql.com/docs/deploy-preview/06003a689142e0e3bbec
Build ID	06003a689142e0e3bbec
File Changes	0 new, 2 changed, 0 removed * (developer-tools)/ios/(latest)/get-started.mdx * (developer-tools)/ios/(latest)/index.mdx
Pages	2

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[pm-dev]

Although this does allow codegen to complete for me, I'm not sure if this is a good idea:
apollographql/apollo-ios#3496

[AnthonyMDev]

I'm not sure that issue is related to this feature. It could be, but I'd like to get more information from you in that thread @pm-dev.