Document OpenAPI model fragment scoping and fix anchor reference

[glen-84]

Summary

• Document the contract for fragment-only OpenAPI definition documents: the first fragment names the shared model, and any further fragments in the same document are private helpers that no other document can spread by name. Also document that fragment names must be unique across every document contributing to one endpoint, since private helpers are not namespaced.
• Fix FragmentReferenceFinder.Find(document, fragment) reporting the anchor fragment as an external reference. The anchor was removed from the local lookup before the document was visited, so a sibling fragment spreading it was misread as a cross-document dependency, and the two Find overloads disagreed about the same document.
• Correct the GraphQL comments in the Shared Fragments examples from -- to #.

Test plan

• New FragmentReferenceFinderTests covering a document whose sibling fragment spreads the anchor; verified failing before the fix (External contained the anchor) and passing after.
• Full HotChocolate.Adapters.OpenApi.Tests suite on net9.0, net10.0, and net11.0.
• prettier@3.8.3 --no-config --check on the changed docs page.

Reported in #10214. Not a full fix for that issue: how multi-fragment documents parse is a design decision still to be made, so this documents and names the current behavior rather than changing it.

[github-actions]

Patch coverage

100.0% of changed lines covered (1/1)

File	Covered	Changed	Patch %
…/src/Adapters.OpenApi.Core/Parser/FragmentReferenceFinder.cs	1	1	100.0% 🟢

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Project coverage: 54.4% (242289/445581 lines)