Avoid double-sending Fastify replies in @orpc/nest

[Copilot]

Using @orpc/nest with Nest's FastifyAdapter could trigger FST_ERR_REP_ALREADY_SENT on successful oRPC routes. The response was being written by oRPC and then sent again by Nest's normal Fastify response pipeline.

• Fastify response handling

  • Change the default Fastify path in ImplementInterceptor to prepare the reply on FastifyReply and return the serialized body to Nest, instead of calling sendStandardResponse(...) directly.
  • Preserve existing behavior for:
    • Express
    • Hono
    • custom sendResponseInterceptors that intentionally take over response sending/return behavior

• Regression coverage

  • Extend the existing Nest/Fastify integration test to capture Fastify logs and assert the request completes without the Reply was already sent warning.
  • Keep the existing assertions around response status, body, and headers to ensure compatibility with current route behavior.

• Behavioral impact

  • Default @Implement(...) routes on Fastify now flow through a single send path.
  • Fastify-specific reply state such as status and headers is still applied before Nest sends the response.

// before
await StandardServerFastify.sendStandardResponse(reply, standardResponse, config)
// Nest would later call reply.send(...) again

// after
reply
  .status(standardResponse.status)
  .headers(StandardServerNode.toNodeHttpHeaders(headers))

return body
// Nest performs the single send

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