[batch-delegate] Error paths use their batched indices

[mjbcopland]

Description

Reproduction test case. When using an n-plus-one delegateToSchema, the result is as expected. When using batchDelegateToSchema, error paths are incorrect.

We expect the path to have index 1 as the error is for the second object, however it is the same error as at index 0 and is deduplicated in the batched request. The error returned has this deduplicated index instead of its actual index.

 FAIL  packages/batch-delegate/tests/errorPaths.test.ts
  ● preserves error path indices › using batchDelegateToSchema

    expect(received).toMatchObject(expected)

    - Expected  - 1
    + Received  + 1

    @@ -28,11 +28,11 @@
              "id": "1",
            },
            "message": "Not Found",
            "path": Array [
              "objects",
    -         1,
    +         0,
              "property",
            ],
          },
        ],
      }

      149 |
      150 |     expect(getProperty).toBeCalledTimes(1);
    > 151 |     expect(result).toMatchObject(expected);
          |                    ^
      152 |   });
      153 | });
      154 |

      at Object.<anonymous> (packages/batch-delegate/tests/errorPaths.test.ts:151:20)

Related #2950

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[ardatan]

Thanks for the PR!
Since we moved the stitching implementation to another repo. The PR has been moved to graphql-hive/gateway#1556 together with the fix. It'll be released soon!