prototype: explicit deduplication clone opt-out

[trippyogi]

This is a design prototype for #433, not an upstream-ready fix.

The investigation exposed a compatibility constraint: eliminating per-consumer deep clones under high-fanout deduplication also eliminates the existing guarantee that each caller receives an independently mutable parsed body.

This branch preserves current isolation semantics by default, removes the unnecessary clone when a request-lifetime request has only one consumer, and prototypes an explicit shouldCloneParsedBodyForDeduplication() opt-out for consumers that treat parsed responses as immutable.

The remaining question is API/behavioral rather than implementation correctness: whether Apollo wants a dedicated opt-out like this, prefers the existing cloneParsedBody override, or wants a different deduplication policy/configuration surface.

Test plan

• npm test
• Default high-fanout still clones once per concurrent consumer
• Opt-out yields zero clones and documents aliasing, including deduplicate-until-invalidated

[apollo-cla]

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