refactor(composition): Remove pub from the state field of supergraph::Supergraph struct

[duckki]

This PR is a minor refactoring. Making the state field private for the supergraph::Supergraph struct.

--- a/apollo-federation/src/supergraph/mod.rs
+++ b/apollo-federation/src/supergraph/mod.rs
@@ -90,7 +90,7 @@ use crate::utils::FallibleIterator;

 #[derive(Debug)]
 pub struct Supergraph<S> {
-    pub state: S,
+    state: S,
 }

I confirmed our downstream repos (like federation-rs, build-plugin-services, etc) are not affected by this change.

---

Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• PR description explains the motivation for the change and relevant context for reviewing
• PR description links appropriate GitHub/Jira tickets (creating when necessary)
• Changeset is included for user-facing changes
• Changes are compatible[^1]
• Documentation[^2] completed
• Performance impact assessed and acceptable
• Metrics and logs are added[^3] and documented
• Tests added and passing[^4]
  • Unit tests
  • Integration tests
  • Manual tests, as necessary

Exceptions

A minor refactoring of an unreleased feature.

[apollo-librarian]

✅ Docs preview ready

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File Changes

0 new, 3 changed, 0 removed

* graphos/routing/(latest)/observability/router-telemetry-otel/enabling-telemetry/selectors.mdx
* graphos/routing/(latest)/operations/subscriptions/overview.mdx
* graphos/routing/(latest)/performance/caching/response-caching/faq.mdx

Build ID: 571e16ddb085b936a3f1b171
Build Logs: View logs

URL: https://www.apollographql.com/docs/deploy-preview/571e16ddb085b936a3f1b171

[github-actions]

@duckki, please consider creating a changeset entry in /.changesets/. These instructions describe the process and tooling.