Fix dictionary value nullability for generated KeyValuePair types

[michaelstaib]

Summary

• preserve dictionary key/value nullability when rewriting collection element types to KeyValuePair<TKey, TValue>
• fix ExtendedType.ChangeNullability position tracking on cache hits so generic nullability flags are applied to the correct type arguments
• infer dedicated KeyValuePair input/output types with field types bound to the inferred key/value IExtendedType
• add regression tests for Dictionary<string, string?> input and output schema generation

Fixes #5470

[github-actions]

Fusion Gateway Performance Results

Simple Composite Query

	Req/s	Err%
Constant (50 VUs)	6929.64	0.00%
Ramping (0-500-0 VUs)	7316.20	0.00%

Response Times & Query

	Min	Med	Avg	P90	P95	Max
Constant	0.57ms	5.92ms	7.07ms	10.99ms	15.80ms	177.31ms
Ramping	0.63ms	24.82ms	30.15ms	63.18ms	75.14ms	193.26ms

query TestQuery {
  topProducts(first: 5) {
    inStock
    name
    price
    shippingEstimate
    upc
    weight
    reviews {
      id
      body
      author {
        id
        username
        name
      }
    }
  }
}

---

Deep Recursion Query

	Req/s	Err%
Constant (50 VUs)	1295.96	0.00%
Ramping (0-500-0 VUs)	1357.17	0.00%

Response Times & Query

	Min	Med	Avg	P90	P95	Max
Constant	4.55ms	34.03ms	37.47ms	51.44ms	60.49ms	352.59ms
Ramping	1.81ms	129.29ms	156.90ms	344.10ms	392.99ms	723.82ms

query TestQuery {
  users {
    id
    username
    name
    reviews {
      id
      body
      product {
        inStock
        name
        price
        shippingEstimate
        upc
        weight
        reviews {
          id
          body
          author {
            id
            username
            name
            reviews {
              id
              body
              product {
                inStock
                name
                price
                shippingEstimate
                upc
                weight
              }
            }
          }
        }
      }
    }
  }
  topProducts(first: 5) {
    inStock
    name
    price
    shippingEstimate
    upc
    weight
    reviews {
      id
      body
      author {
        id
        username
        name
        reviews {
          id
          body
          product {
            inStock
            name
            price
            shippingEstimate
            upc
            weight
          }
        }
      }
    }
  }
}

---

Variable Batching Throughput

	Req/s	Err%
Constant (50 VUs)	23532.00	0.00%
Ramping (0-500-0 VUs)	18406.36	0.00%

Response Times & Query

	Min	Med	Avg	P90	P95	Max
Constant	0.08ms	1.73ms	2.08ms	3.99ms	4.83ms	38.22ms
Ramping	0.08ms	9.44ms	11.47ms	23.66ms	28.50ms	127.42ms

query TestQuery($upc: ID!, $price: Long!, $weight: Long!) {
  productByUpc(upc: $upc) {
    inStock
    shippingEstimate(weight: $weight, price: $price)
  }
}

Variables (5 sets batched per request)

[
  { "upc": "1", "price": 899, "weight": 100 },
  { "upc": "2", "price": 1299, "weight": 1000 },
  { "upc": "3", "price": 15, "weight": 20 },
  { "upc": "4", "price": 499, "weight": 100 },
  { "upc": "5", "price": 1299, "weight": 1000 }
]

---

Run 22394727810 • Commit 1baf19c • Wed, 25 Feb 2026 13:18:41 GMT