Deduplicate external fragments in near-operation-file preset

[Nurou]

This deduplicates externalFragments by name in near-operation-file when multiple source documents are merged into the same generated output.

Why:

• multiple documents can resolve to the same generated file and each contribute the same external fragment
• downstream plugins then receive repeated externalFragments, which can cause severe perf blow-ups

This is related to #752 and supersedes the narrower consumer-side fix I initially opened in dotansimha/graphql-code-generator#10728.

Validation:

• added a regression test that merges two documents into the same generated output and asserts externalFragments are deduped

Downstream benchmark from the original report:

• before: ~248s
• after dedupe: ~2s

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