Deduplicate external fragments in typescript-operations

[Nurou]

This deduplicates externalFragments by name in @graphql-codegen/typescript-operations before they are passed to TypeScriptDocumentsVisitor.

Why:

• near-operation-file can supply repeated nested externalFragments
• typescript-operations currently forwards them as-is
• this can cause severe blow-ups when repeated fragment spreads are nested

This is related to dotansimha/graphql-code-generator-community#752.

Validation:

• added a regression test that asserts repeated externalFragments are deduped before visiting documents
• yarn workspace @graphql-codegen/typescript-operations vitest run tests/ts-documents.externalFragments.spec.ts tests/ts-documents.externalDocuments.spec.ts

Local benchmark from a downstream repo using near-operation-file + typescript-operations:

• before: ~248s
• after dedupe: ~2s

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